Most online knife dealers sell them. But you're going to need to spend at least $70 to get a decent one.
This one is the best bang for the buck. Only $70 but flips just as good as balisongs costing four times as much. Any of the Bear Ops balisongs (
not the Bear & Son balisongs, same company but very cheaply made, although if you can only spare $35, a Bear & Son is probably your best bet). The other Bear Ops balisongs have different appearances but are functionally identical. The fancier ones obviously cost a bit more than the $70 plain black one.