Check out http://AquaHub.com. Good prices on their own and also Madison brand. If you are looking for a pre-built system, check out http://AutoTopOff.com for customized units. I used aquahub float switches for my topoff system, but if I didn't already have an Aquacontroller 3, I would probably have bought a topoff unit from AutoTopOff.com and customized it to my needs.
Oops - I see that you are not looking for a top off system. I would suggest just looking at the Aquahub site.
I am also hoping to implement an automatic water change system soon. I would much rather do that than the ATO. I just need to figure out how I am going to implement a overflow in my sump, which is a 55g aquarium. I should have had it drilled before I installed it. I am kicking myself in the butt! Oh well. Good luck, and it would be nice if you were to make a new thread with some pics of your progress!!