There's a guy on the NCPARS forum that has an Aquatek 6800 for $40 shipped. He only has the pump, no transformer or valves, so check your transformer for power.
I'll PM you his info. I meant to check it out at their swap yesterday, but completely forgot.
I'm also looking for a RO Booster, but I need everything (pump, transformer, pressure switch, and shut off valve)