I think your best bet is going to be to find them online. Loganj just got some from someone on ebay and was pleased with his purchase, you might look for his post in the general retailers forum. As for how many...I have heard it suggested that you should have one per gallon, but I think that might be excessive. I only have 4 in my 20g and they do a good job keeping the excess food eaten. I would start with 10 or 20 and if you think you need more, go from there