You can buy tablets online. in fact, I don't know what shop in the street would sell them anyway :P
I don't know what websites are best, sometimes Ebay will have a second hand one if you are used to using Ebay its worth a shot. or the manufacturers website of that particular tablet, such as wacom etc.
cool robot designs. the images are really scratchy with lines so its hard to make out details. have you tried using vectors instead, like using Inkscape, or using paths such as in GIMP? I think you would be able to have a bit more control that way, even with a mouse.