Two online sources are ULine and Cheswick (the latter is part of Staples).
You may also find what you want on eBay. I buy the poly envelopes I use here, but when I use B-Flute (a kind of single sided flexible cardboard, available as a selfstick product) I get it from Cheswick.
Look for the boxes, if that's what you are sure you need, that make up into multiple sizes. Not sure what they are called, but browse on the websites and you will find them.
Oh, if you order from the States, the prices may seem good, but if the seller ships by courier, you will be charged a healthy fee ($25 or more) in addition to the sales tax and duty that you will always pay. The seller is likely unaware of this, so you should be.
