Which isn't all that aesthetically pleasing.
So here is a list of what I've found so far that seems useful, I'll update this post as I find more, and if you've had any experience (good or bad) with any of these companies, please comment:
Paperworks.com - pretty much a paper site, but they seem to have a nice variety of coated, photo, and specialty papers and envelopes. There's also a section to order samples, which might be all you need for a backer, header or insert.
Papermart.com - I just found this one and it seems like there's a ton of useful items here. All sorts of gift boxes and specialty boxes, including non-square shapes, tins(!), shred, and cello bags. Pretty much anything you can think of in the packaging department, they have. Including mops. For cleaning up, um, packaging disasters. SPLODE!
Uline.com - The old standard for bulk cheap boxes, though they seem to cater more toward larger boxes for moving and the like. There are some smaller boxes though. And they have a nice amount of poly bag supplies, including sealers for custom-size bags.
Royalmailers.com - standard bubble and flat rigid mailers (good for photos or prints, which I'll probably pick up for shipping my prints), seems more catered to flat and document mailing, though they have boxes as well.