Several manufacturers are moving away from 7-inch tablets, but LG still has the small tablet form factor in mind. The G Pad 7.0 came out last year, and eventually found its way to AT&T. Now you can grab that device for just $109.99, and it's fully unlocked.
The G Pad 7.0 (model V410) is a mid-range device, but it might serve your needs if you know what to expect. Here are the specs.
These devices are new, but have been opened in order to be unlocked. The LTE will work on AT&T and HSPA+ should be functional on T-Mobile and other GSM carriers. It sounds like the v410 also has band 2 LTE for AT&T, which T-Mobile is rolling out in some places. You might have luck with that, but no promises.
You can get this deal from eBay or Groupon, both of which are offering the $109.99 price with free shipping. eBay will charge tax in California, though.