I got mine from FilterBag (here's a
link). You have to get 25 at a time but I paid $55 U.S. for 25 of them including freight. I change mine every 3-4 days and just take the dirty filter and put it in a bucket of bleach/water for a couple of days. Then it rinses out pure white. Just leave it to dry and it's good to go. I have about 12 going in a cycle and only use 2 at a time so they get a good couple of weeks to dry before being reused.