Personally I would avoid dunking an aquarium heater into fermenting beer since I always feel, rightly or wrongly, that you are slowly cooking it off the hot heater surface.
So for me a much better idea is to use the water bath as described here How to Set up a Water Bath for your FV - The HomeBrew Forum
I bought my 50w heater off ebay some time ago and its still functioning
I think they are probably all much the same. I remember someone on here saying he had bought his heater from the local aquarium shop (garden centre or pet shop?) on the basis that he could take it back if it failed early.
Mine off ebay is one of these (50w)