Here's a pretty comprehensive list:

http://en.wikipedia.org/wiki/List_of..._UMTS_networks

The spectra around 2100MHz is extremely common (pretty much every carrier on that list sans AT&T, T-Mobile, and a handful of others. ). The iPhone 4S supports 2100MHz, as well as 850, 900, and 1900 (AT&T).

The only band that doesn't work is 1700MHz AWS for T-Mobile, so until they're finished with their 1900MHz deployment, all you can use is EDGE.

Like D582 said, the real issue is whether or not you can obtain a prepaid microSIM. You can always cut down a SIM (assuming you don't have to return it), in case you only get the normal-sized one.