Best reasonably priced float switch

Depends on if you want the float switch to directly control the ATO pump, or if it fires a relay. The cheap ones can only handle half an amp or so at low DC voltage, so you have to use them to control a relay (which is what I do).

I have some cheap ones that I ordered from china, and they've been working fine for me, as long as you don't pass a lot of current through them. I have mine firing a relay that activates a 12V bilge pump designed for use on a boat (like $12), it works for me.