Well, two things to check...

You could always search for vending machines on ebay. I am sure you will find something.

Another possibility is to contact the distributers to see if there are any old machines they have or want to sell. Similar to what they recommend in the arcade auction and purchasing FAQs. Many of the same principles apply. Here is a Google search that may help a little. Also, Happ Controls, the same folks who make the video game / MAME parts discussed elsewhere also make the parts for the vending machines, and sell complete machines (retail price, though).

Hope this helps a bit.