EBay is your friend. I have like 7 various heaters I use.
and some random one I got from a seller in China.
The only one I DO NOT
recommend are the Tetra preset ones as they dont work for crap.
Top Fin - I have had no issues and mine came used but they eem to have a reputation for cracking in the water?
The Neptune one was my fav and was always spot on but sadly it got craced when I moved out of Alaska last year
I dont even thin they mae them anymore
Hagen Elites are nice becuase they are only like 5 inches or so tall so they fir well into smaller bowls/tanks. There are aparently 2 kinds. One has a clip that attaches to the tank lid and I beleive is the older version. The there is a submersible version that says its "preset" but it is actually adjustable.
Aqueon seems to work well as well as the random one I got from Asia. Just make sure if you get one from Hong Kong/china/tawian that it is in the right voltage as the sell both US and Europe versions.
Marineland Stealth - not sure if they still mae these either but it keeps the temps pretty good. Drawback is that it is a tall heater and will barely fit into the larger Kritter Keepers. It has a hard plastic shell around the whole heater so it wont shatter BUT there is no light that goes on and off like the other brands
OOOH the one Aoashi posted looks like my random heater I got from asia :D Except mine is 50 watts