Well, I have a 15w hydor mini-heater (5-10g) that I could use, but I'm concerned about using that one in my 1.5g because it doesn't shut off. I have the 7.5w mini-heater in another 1.5g tank, and it works 'fairly' well. I'd get another one of those, but they don't sell them here in Newfoundland. I bought it at Big Al's in Ontario when I was on vacation last year.
The problem is that I can't really afford to get a low wattage heater because I find that, especially here in Newfoundland, they're really expensive. The Marina compact 10w heater (preset), is about $30 (Canadian). I bought one on sale last week for my female betta's 1.5g tank, and I can't really afford to buy another one. The 25w Tetra heater is only $12.
I'm considering taking the 10w one out of my female's tank and putting it in my new fish's tank, since she didn't seem to mind it being too cool and was really active without it, but my new guy has gone all blah and hardly swimming at all since I put him in his tank, which I find the male bettas do if they don't have heat. He was really active last night when I put him in, and today he's barely moving.