Jason Google cartons boxes, etc and your location, and I'm sure you'll get some suppliers come up to contact.
Try to get to the manufacturers rather than the middle men . . . here in Aus it is a very competitive market.
Consider this when choosing sizes.
Get the width and depth you want, then order the heights longer.
It's easy to cut these down to suit (you can even get them kinda crimped every 3" or 4" or whatever, so you can fold them wherever you like), and you will lessen the number of sizes you might need to keep (purchase, store, etc).
For example, for 22 x 18 and 24 x 20 framed sizes can go in the same cartons, but you could also use for 20 x 16, and just cut down the 24 length to suit 16.
Put your printed logo etc (if applicable) in one of the lower corners each side so it doesn't get interfered with when trimming.
The cost difference of these sizes makes it feasible for the 'wastage' in my past costing experiences, just use the offcuts for extra corner reinforcing etc, or at least recycle this.
I'm sure you'll find good solid corners too (maybe from the same supplier), and make the carton thickness to suit the corners so the art is isolated central in the carton.