Any good 25 watt heater will work fine in a 2.5 gallon. Personally, I've got a Hydor "Theo" heater-- it doesn't take up too much room and it keeps a steady temp in a small tank.
Yes, it really is one of those great moral dilemers (sp?). If you can afford it, I'd buy a fish off aquabid from an American dealer. Then there's a good chance it will be home bred by actual fish-lovers.