I of all the irony use Staples. and no I"m not realated to the guy who invented staples. lol. Trust me I would have hit him up sooo long ago. lol
My store has a good selection of plain white and natural boxes with all different shapes and sizes. Mine come in a package of 8 and there like 9 dollars so there a little over a dollar a piece. But I like that its consistant, if I print out labels by paypal then I know how much it weighs etc.
I reuse boxes too as long as they are in good shape. My mom orders a lot of books, So if the box is still in good condition and the approprate size I will use it.