If you have to ask where to buy them from, I wouldn't buy them. I don't say this to be a smart-aleck, but I think most people who have success with them already know where to find them and are a part of that circle.
It is true that there are many tropical ones, and even a few tiny ones, but feeding and flow are often difficult to recreate. Plus, as cephalapods, they need high O2 in their water because they do not use hemoglobin, but instead utilize a much poorer oxygen receptor in their blue blood.