You can buy it from what I've seen at Marine Depot or directly from the link above. I'm going to buy the full Apex here in the coming weeks. Figured I'll use the variable speed pumps some day, want the PH probe, and would like the second one too.
Aquacave has them too. I was wondering about people's experiences with them. Good, average, bad or whatever they have to say. I guess I'm just wondering if I should go the economic rout or go balls to the wall