I use one of
these. Aside from having to clean the feather dusters off of it from time to time the thing appears to be bomb proof. Had mine going for over two years now. I liked it so much I bought a backup unit just in case.
If you can find float switches locally you can make your own quite easily. I was lazy so just bought this pre-made one instead. The second cut-off float is handy too in case the first one fails.