I bought mine from Peke A Tzu Rescue (it's a small breed, non-profit rescue).
http://www.pekeatzurescue.com/bellybands.htm
I'm not sure if this is the best place to get belly bands. I just purchased them last week and received them yesterday so I haven't used them yet. They had so many different cute patterns. Also, 100% of the belly band sales go to the vetting fund so I felt good about ordering from there.