Do you know anyone with an iPhone 4/4S on AT&T? Maybe you could borrow their SIM.
Other than that, I know T-Mobile sells a microSIM, but I'm not sure if it can be activated on prepaid. As for AT&T, I'm pretty sure their microSIM is for postpaid only.
If you're willing to get a regular SIM and cut it yourself, you can try getting a T-Mobile SIM and adding $10 to it. Or if you might be able to find one on eBay that either has an initial balance (like free 10 minutes for setting up voicemail and stuff) or one that's already been activated and has a low balance left over.