You have a lot of options for ATO. You can buy rigs like the Tunze ATO which a lot of stores sell. You can rig up a float valve (like the kent float valve) to refill your sump from an RO/DI unit. You can MacGiyver a float switch like
this, or you can just buy a float switch ATO system like
this.
I use the last one from autotopoff.com. I tried very hard to find float switches in the city but was not able to.
K.