Difficult to say whether Hangzhou or Suzhou is better. In Hangzhou, there is a place called China Silk City (中国丝绸城), which is a street selling various kinds of silk products. I think you could have a try there. It is very close to the Wulin Square. In Suzhou, you could find many silk shops around these attractions, such as the Humble Administrator's Garden. No matter where you purchase goods, please remember to bargain the price.

For tailors, I think you could go to Suzhou, such as these ones at Guanqian Street and Huqiu. Usually, at these silk shops, you could find a tailor, too! A summer dress may costs you about 300~500RMB， including silk and tailor fare.

to be a hangzhou guide,i can not easy to tell you about silk where is good. in suzhou, silk have long history . bout you can see, all of the world, about 80% silk is in china,among them, about 60% is come from zhejiang, hangzhou is must for the best.