Any bug warehouse store like Sam's Club or Costco usually has plenty of large flat sheets of cardboard that can be used to make your own boxes if you sell a variety of different sized items, though they also have a good assortment of boxes for customer use. A lot of other places also throw away completely good boxes and packaging, so you should never really have to pay for stuff like bubble wrap. As others have said, if your items are heavier than 13 ounces and are of a reasonable size, USPS offers free priority mail boxes and padded envelopes if you are going to pay for priority shipping anyways. The only supplies I ever seem to buy is tape and the occasional small bubble mailers & envelopes.