Melissa, Warm Welcome to FT!
Why Singapore to get a prom dress done? When would you be coming to Singapore? If you are coming in November/December, it's the "prom season" in Singapore and most department stores would carry a wide range prom dresses in their stock.
You can try Daniel Yam in Wisma Atria which have very very nice dresses. Here's their website, but it's not updated:
www.danielyam.com
Allure in Raffles City has a slightly more oldish range, but still worth a look.
I know I am not answering your question, but hope this helps anyway!