Used is fine.
If you do want to use new boxes, and remember if you are gleaning used ones they are still costing you time if not money, don't buy them at the Post Office.
Those boxes are priced as 'convenience' items for occasional shippers who need it now no matter the price.
ULine is an online source we've used. Staples also sells boxes both in their stores and online.
Both also carry specialty products for shipping CDs, DVDs, photographs.
There is an interesting product called B-flute which is a 'make your own box' item. I like the kind with gum.
And if what you are selling is flat and needs 'stiffeners' rather than a box, look out for a corrugated plastic board called 'corplast'.