There's two in Sanctuary itself, one is also out in the desert (Dust) in a half buried shack and I saw another one on the back of a bandit in a area once, aptly called one armed bandit (And yes he only had one arm, similar to those Loot Goons but rarer.) but I think the same chance goes for any of them, at best I've gotten the Eridium jackpot at Sanctuary (That stack of three Eridium bars in a line.) would be interesting to see the others but I've tried hundreds of times and it won't really lead to anything.
Slot machine hack doesn't seem to work for me. It changes the three symbols that come up on the machine to whatever I chose but the actual prize still seems to be random and doesn't match the chosen symbol.