A long time ago someone suggested using those tiny ziplock containers. I bet you could get them at the dollar store. Someone also said that baby food comes in those now? So maybe if you know someone uses that kind of baby food? Try
www.papermart.com or
www.nashvillewraps.com for box options.
Also if you are just shipping a FEW right now, they do have the boxes at Michaels/JoAnns but they are pricey for the 6 or 8 that you get. But, maybe it's worth trying esp. with a 40% off.