I actually got mine from petco.com. They were having a sale where if you ordered more than $45.00 you got 10% off, plus shipping was cheaper than on other sites and when you added it all up, I got 3 full units for $75.00, which was the cheapest I found on all the sites listed....at the time. They may not be running the sale any longer, so you just need to check out the various sites.

HOWEVER, I found that it does NOT pay to buy them in the store! Both PetSmart and PetCo had them online for about the same prices QSA listed above, but when I went to each store, they were both around $40.00 for the full unit and $25.00 for the refills - WAY more expensive than online.