I'm not sure where your best bet is to buy them Hank but there are lots of options. I'm also not sure if they are all created equal. Lee Valley sells 6mm and 8mm from Canada. I think Rockler sells them in the US. LV's are $5 Canadian. I think Rockler's are $4 US so roughly the same price or a slight edge to the Canadian ones. 10mm is supposed to fit a 3/8" collet but I haven't tried it yet.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.