I went though this before on other sites and you can try the happycow-site for fining things you need.

Many thai are vegetarians. The word for vegetarian is AHAN JAE (say it as jay) and if you use the word jae after a dish they will know without meat and will ask if you want KAI (egg). Veggie fried rice is Khao Phat Jae.

Since many Hindu-Indian (vegetarian) visitors go to Thailand it is VERY easy to get veggie food. I think you can also use the thai word MANGSAVIRAT to say you are veggie.

A list I was given... not created by me... is below...

The Whole Earth; 93/3 Soi Lang Suan, Ploenchit Rd.; 12 mins walk; Thai/Indian
02-252-5574; 11:30-14:00 / 17:30-23:00 Thai and Indian vegetarian/vegan dishes served.

Bawarchi, Hotel Intercontinental, 971 Ploenchit Road Indian & Zen Indian Chinese;

Food Centre, Central Department Store, 1F Central Department Store (TOPS), Chitlom Thai/Chinese Vegan. Vegetarian vendor at this food centre. 10-15 kinds of Thai/Chinese dishes and one soup noodles. Red rice.

Food Loft, Central Department Store, 7F Central Department Store, Chitlom Indian and Italian booths have dishes such as veg curry, dahl curry, veg samosas, tomato sauce pasta, Genovese pasta, margarita pizzas, veg soup

Amarin Plaza, Ploenchit Road ThaI
Food court, 3rd flr; 10-17;

Food City, Big C; 4F, Big C Ratchadamri Road Indian/Italian
Indian vendor called Mughal-Darbar at this food centre. Some vegetarian food from Spicio, Italian fast food place is available, too.

Mah Boon Krong Food Centre Top floor, 444 Phayathai Rd. Thai/Chinese
Food Centre, Vegan 10:30-21:30

Nova Kitchen, MBK 6th flr Phayathai Rd.; Chinese,
11-21

Ploenchit Center Sukhumvit Soi 2, 6th flr Thai
Veg food stall in food court

Al-Humaidan 4 Sukhumwit Soi 3 Egyptian
6555380; 10 - 02 . Arabian salads, hummous, foul.

Al-Husaini Sukhumvit Soi 3/1 Indian, Arab,
07-03

Nasir Al-Masri & Shisha 4/6 Sukhumvit Soi 3/1 Egyptian,
07-04

Bollywood, Sukhumvit Soi 4 Indian;
150m on rhs

Bukharah’s, Sukhumvit near Soi 7 Indian

India Today, corner Sukhumvit Soi 8 Indian
Above tailor shop.

Indian Garden, 15-17 Sukhumvit Soi 8 Indian
0-2653-3565; lhs;

Bawarchi, Am Plaza Sukhumwit Soi 11 Indian
10-02

Mrs. Balbir’s Sukhumvit Soi 11/1 Indian
Cheap

Dosa King ; 265/1, Sukumvit Soi 11/1, Indian,
Vegan-friendly ; 02-6511700; 11-23

Pasand, Sukhumvit Soi 12 Indian;
above Value Mart on corner soi 12;

Cabbages & Condoms, Sukhumvit Soi 12 Thai
11-22; 02-2294611

Crepes&Co ; 18/1 Sukhumvit Soi 12 French / Mediterranean ;
02-653-3990-1; 09-24 Some crepes are vegetarian. 400m rhs.

Sunrise Tacos, 236/3-4 Sukhumvit Road, near Soi 12 Mexican ;
Near Times Square; 02-229-4851; also vegetarian burrito’s

Rang Mahal ; 26F Rembrandt Hotel , Sukhumvit Soi 18 Indian;
02-261-7100; 11:30-1430, 1830-22

Shri Ganesh Restaurant, 392/23-24 Sukhumvit Soi 20/1 (between soi 18 and soi 20) South-Indian;
02-258-1425

Tamarind Cafe, 27 Sukhumvit Soi 20 Thai/Western
02-663-7421, 11-24 Expensive

Komala's; 15 Sukhumvit Soi 20 lhs, halfway down South Indian
Vegan; Fast food style South Indian. Nice and cheap food like dosa, veg burger, samosas. 02-663-5971; mon-fri: 11-22; sat-sun: 10-23;

Govinda , 22 Village, Sukhumvit Soi 22 Italian
02 663 4970; 18-24, closed Tue;, Vegan-friendly; All veg pizzas, pasta, mains and desserts! Mock chicken dishes

Lemongrass, 5/1 Sukhumwit 24, 2588637
Vegetarian meals on request. Opp. Emporium.

Kwan Imm Jae, 6/15 Sukhumvit Soi 24/1 Chinese
02 259 6466; 11-14 & 17:30-22:30

Indus 71 Sukhumvit Soi 26 (nr. Four Wings Hotel) Indian
02-258-4900; Expensive and very cosy Indian.

Hazara ; Sukhumvit soi 38 BTS Thonglor / 5 mins walk ; Indian;
02-713-6048; 18:30-23:30 Located at the Face (Face Bar, Lana Thai Rest. and Spa). Very nice atomosphere and delicious food.

Rasayana 57 Sukhumvit Soi 39 , Soi Phrom Mit, sub-Soi of Sukhumvit Soi 39 Raw Food Cafe;
Vegan, Organic; B100-200/dinner; 02-662-4803-5; 09-20. Outdoor cafe.

Lemon Farm Sukhumvit Soi 39 , Soi Phrom Sri, sub-Soi of Sukhumvit Soi 39
When reaching traffic Lights in soi 39 turn right (opp. street). 02-2041056

Big John’s Home Baked Pies & Guesthouse 159/12 Sukhumvit Soi 55 (soi 9 lhs) Thai, International;
02-392651; Mashed potatoes, pea soup, veg. Spaghetti, tofu burger, mock meat; 07-02

Tienjang Jae Food, 159/15 Sukhumvit Soi 55 (soi 9) Chinese/Thai;
02-382-1242; Small place; friendly; Mock meat dishes; Just before sub soi 9 A few shops in the corner.

Chijuya, 333/4 Sukhumvit Soi 55 (soi 17) Japanese, Taiwanese
02-259-6466; 2nd flr, Fifty-Fifth Plaza 11:30-14:30 / 17:30-21:30, Vegan-friendly; Delicious Japanese and Taiwanese veg food. Sushi, tempura, many mock meat/fish dishes, etc.

Mango Tree, 37 Soi Tantawan Surawongse Road,
02-236-2820. Separate pages in menu for vegs. Good atmosphere.

Bio Kitchen & Bakery, 425/15 Soi Phipat Silom Thai and international.
11-22:30

Namkang 5/3-4 Soi Phipat Soi 3, Silom Korean
2331480/2331483; 10-14 & 17:30–22. Variety of veg dishes like kimchi, seaweed soup, fried tofu with spicy dip, fried nori, pickles etc.

Aim-Boon Vegetarian Restaurant, 1497 Rama 4 (at Soi Chula 11 corner, nr. Th. Phayathai) Vegan, Chinese
02-2154296; Open Tue-Sun 09-19 closed Monday.

Suki Jeh Ru , Rama 4, 285 Soi Phraya Singhaseni Street, Chinese, Thai
(Opp. Bangkok Centre Hotel, north of Rama 4 Rd. near Hua Lampong railway & MRTA station)
02-2191721, 086-0689384. Open daily 09-22, except Wed from 17:30. Subway to "Hua Lamphong", exit 3, backtrack on Rama 4 (head east) for about 70 m. to Soi Trokphrayasinghaseni, turn left, restaurant is on the right with a sign in English, "Vegetarian”.

Yogi, 62 Soi Putha Osoth Surawongse Road Indian.
02-2361809. Closed Mon; 1130-21

Chennai Kitchen, 52/24 Thanon Pan Silom Bangrak South Indian;
02-2364299; 10-15; 18-2130

Mashoor, 38 Thanon Pan Silom Bangrak South Indian;
02-2349304; About 6 doors from the Indian Mariammam temple.

Slimbo, 29/18-21 Soi Sunvichai, Thai/Chinese
02-2030917. 11-24.

Anotai; 976/17 Soi Rama 9 Hospital, Hwaykwang MRT Rama 9 Thai, Organic,
Vegan-friendly ; 02-641-5366-70; 11:30-21:30 Closed Wed. Hidden place on west side of Rama 9 Hospital. Organic veggies ! Thai dishes, fruit juice and desserts.

Om Boon Vegetarian Food, 130/26-27 Phayathai Road (nr. Phayathai BTS)
02-2156988; Mon-Sat: 10-14;16-21; Closed sun; Extended hours during vegetarian festival;

Sirukul Mangsavirat, Tra Jae Food Centre next to Vanich Building TV3, New Petchaburi Road Thai;
02-9825094;

Robinson Department Store (Tops), Ratchadapisek Road Thai
Vegetarian Food-stall in Tops Supermarket - basement; 11-22

Taketei; 117/1 Tanao Rd., Banglampoo, near Khao Sarn Rd. Japanese
Aircon bus 511&512, ordinary bus 2,39,10&12 ; B50-100/dish ; 02-932-86731; 11:30-25:00
Go there if you want cheap Japanese vegetarian food in Khao Sarn area. Watch out for fish stock (katsuo-dashi) !

May Kaidee's Vegetarian Restaurant ; 117/1 Tanao Rd., Banglampoo, near Khao Sarn Rd. Thai,
Vegan-friendly Aircon bus 511&512, ordinary bus 2,39,10&12 ; B50-60/dish ; 02-932-86731; 8:30-22:00 Located at the back of Burger King. Standard Thai dishes such as Pad Thai (fried noodles), curries, mango desserts, etc. available with reasonable price.

Cafe Corner Samsen Road, Soi 2 (near Khao Sarn Rd) Thai, International,
Vegan-friendly ; B100/lunch or dinner ; 01-3424755; 07-25; Relaxing cafe, Thai and light Western meals.