You can make one like the above for <$10. I don't recommend them though because there is 120vac being run through the switch. Not that the switch can't handle 120vac, I just don't like sticking it in my tank if there is another way.
I would recommend a 12vdc wall power supply, a relay and a switch set up. It's all low voltage and no fear of the magnetic effects of using ac.
The dc version can still be made for not much more than the ac version. You just need a power supply and a relay extra.