Have you looked at Kelco?

Mitchells is probably your best bet unless you are looking for thousands, then cherokee.

check glamos wire if you need a lot of frames at once.
If you just need a few at a time, check out
northwoodsevergreen.com or fraserknoll.com

I don't know what brand my clamp machine is, I got it used, but it takes a standard single clamp. Hopefully your mitchell is the same. both of those places have good service and turn around times, and you can place small orders.