Well, there are actually some heaters that are good for tanks that size. Such as...
http://www.bigalsonline.com/BigAlsUS/ctl3684/cp18534/si1382458/cl0/hydortheosubmersibleheater25watt
You can order this one online, but even though it's reccomended for your tank size, I still advise you to set the temp a bit lower then you want it at (Around 78).
Until you get it, just keep a thermometer in there and I guess add some warm (not hot) water every so often. If you're putting in 0.5-1 gallons of water, it isn't that big of waste... Plus it's going to a good cause.
Good Luck