Why not ask Warmoth? If anybody has them, they certainly do. I used to try to keep some around, but storage space used to be more precious than it is now so I tossed most of them. Those that are left have occupants.
For full-size guitar boxes to ship cases/guitars, the local guitar resellers (Musician's Fiend,
et al) throw away more than they keep. Ask them and you'll probably not only score, they'll be happy to clear them out.
The UPS Store will pack things for you for a nominal fee. Not the
best solution, but if damage occurs it's on them for poor packaging/handling.
For necks, the USPS has pretty much the same boxes Warmoth uses...
I'm pretty sure those are somewhere between free and cheap.
Boxes aren't bad, price-wise, regardless of the source. It's the packing material (styrofoam sheets, bubble-wrap, etc.) that's killer for some reason. I have several garbage bags fulla that stuff left over from the various things that show up here. Always save that.