Go to the electronics market on Xiangyang Rd. @ Fuxing Rd. I have a WCDMA Unicom SIM that cost me y300, and is good for 6 months (data only; i.e. no phone number attached). In your case, I believe they sell CDMA SIMS (a little slower) for y100 that you just burn through and toss (these have phone numbers, as well).

While going to the Xiangyang Electronics Market may be annoying for you, this approach is FAR superior to visiting a Unicom store (just trust me). A newspaper stand would be the easiest, but none of the ones near me sell data SIMS (well, they do, but you need to go to a Unicom store activate a data plan, which sort of defeats the purpose). Furthermore, while you're at Xiangyang, you'll have an opportunity to pick up some random gadgets (e.g. they sell USB chargers that support every device under the sun for around y25).