This is the one I use:
http://www.jlaquatics.com/phpstore/s..._ID=ro-kpfshut
Thing is, especially with a mixing station, you're not going to be filling it 24/7, only when you want it. So it's more of a "in case you forget to turn it off right away" thing. Fill the reservoir, let the float valve shut it off so it doesn't spill over, and then when you come check and see that you're done, turn off the water anyhow.
I run float valves on all my tanks off a reservoir that I fill about every 2-3 weeks. The reservoir itself has a float valve and that kicks the RO/DI off using this solenoid when it's full. So I just turn the RO on, let it fill, next day I turn it off when it's done.