Hiya! Tortoises are great pets. You can get them from reptile shops, just be sure to research them before hand, unfortunately you still get plenty taken from the wild and they never do well in captivity so getting them off a place like gumtree would be a bad idea.
A Hermans or a spur-thigh would be your best bet, they're the easiest to look after and don't get too big.
I would ring a specialist exotic vet for some husbandry advice when you get the little thing (I work at one if you want that, though I am in London). Pet shops aren't always up to scratch with their advice, that's all
Enjoy him though, they're fascinating little things