I highly recommend FeiFei tailor shop. I have had many things made there. It is a small, privately owned shop (no sweat shop labor). Ask for the shop owner Xiao Fei. The shop is located at:
35-1-103
Xin Yuan Jie, Beijing (near Capital Mansion and Kunlun Hotel)

FeiFei doesn't stock a lot of fabric, but they can get any fabric you need, or you can supply your own fabric. Of course, asking them to get the right fabric is easier if you speak Chinese.

If you want to buy your own suit fabric, the cheapest place is the fabric market in Tianjin. If you have a free day, you can take the train there (takes a little over an hour). If you can't leave Beijing, I recommend Yue Xiu market on ChaoYangMen street. But, be prepared to bargain heavily if you go there. Yue Xiu won't be as cheap as Tianjin, but the price is still reasonable after bargaining and the fabric is good (but I don't recommend the tailoring service at Yue Xiu). I would avoid getting your fabric at the market in WangFuJing. This place is loaded with tourists, the prices are horrific, and the tailoring quality is questionable.

The price will vary depending on how much you spend on fabric. An average man takes about 4 meters of fabric to make a suit (safely). For nice 180 thread count Australian wool, the fabric should cost about 65 yuan (US$8) per meter in Tianjin, or about 85 yuan (US$11) per meter at Yue Xiu. Tailoring cost is usually about 400 yuan for a decent tailor. In total, you should pay between 600-800 yuan (US$75-$100) per suit depending on the fabric quality and how good you are at bargaining.

The tailors can make any suit you want, with any features. They are also very good at copying suits from pictures, so if you have a picture of the suit you want, bring it with you. They can also arrange to have the suits delivered to your hotel.

There are cheaper tailors in Beijing, but I wouldn't recommend them if you are wanting nice suits. Some people may recommend the fabric market at MuXiYuan in south Beijing, but I went there recently and found that there was very little suit material for sale.