I'd start by researching RO top offs at Reef Central to get the complete list of parts that you will need. The whole set up is going to be expensive. That is why I was suggesting another float valve as a failsafe for you looking after the filling.
For the solenoid and float switchs, start by looking for surplus stores in your area. Secondly, do an online search for surplus stores and try eBay. If all else fails, buy new parts online. I'll look into it later on and post what I find.
