If you dig around on autotopoff.com's site they list parts and sell the float switches pretty cheap.

Personaly I like these guys http://aquakit.ca/ they have done well bye me in the past as a matter of fact I was just working on my own DIY ato using their parts.
I found them to be well priced aswell

I just finished a DIY ATO on my 72 gal with 25 gal sump , it cost me about $30 & an hour & works great . I use bottled RO water for top up so I just drilled a bottle top , inserted 2 - 1/4" clear vinyl tubes.
1 is connected to an Aqua lifter pump that sends the water to a 2 way valve on the sump then 1 side of the valve is connected to a $6 humidifier float from Home Depot , the other side of the valve acts as a recirc line back to the water bottle when the float shuts off. When the bottle is empty all it takes is a cap swap & its good for another 3-4 days.