What you are looking for is usually known as a 'receipt printer'. A quick Google for 'bluetooth receipt printer' turns up a number of models from a variety of manufacturers (quite a few of them are actually battery powered and portable - like you see at car rental places).
The big problem here is that Android doesn't really have any concept of local printing, so I think you're going to have to implement it yourself.
Android DOES have bluetooth communications APIs (docs), and I'd be surprised if the printer manufacturers haven't heard all this before, and have at least clues how to proceed (after all, they like to sell printers!). If you'll be buying a significant number of printers, I bet they'll happily help you talk to their devices.
If that's too expensive for you (and those printers look a bit pricey), then you could try USB. Again, you can find USB receipt printers, which again you'll have to learn to talk to. Android does have a set of USB APIs (docs). On the plus side, my quick Google for 'USB receipt printer' turned up some sub-$75 choices right away, so this might be a good approach if you are price sensitive. I'd also be willing to bet that at least some of the USB printers are actually serial devices with something like an FTDI chip bolted up to them. That means that talking to them is probably not very difficult.
Alternately, if you can find a cheap receipt printer that's serial or something, you could look at the IOIO boards and software stack as a way to interface your android tablet to some funky printer. These are boards that adhere to the relevant Android APIs + software that makes them easy to talk to. The boards are available commercially, and the schematics are available, if you want to build your own, and they are designed to make hardware interfacing easy.