At most any fairly large coin show where a grading company is represented.

Ocassionally after a large show is concluded, like ANA or FUN, sometimes these empty boxes are laying on/or behind dealer's tables who have packed up and left. Lots of coin auction houses use them to show lots to prospective bidders. I'll bring one with me to the A.C. Chapter Meeting this Sunday to show you if you are coming. Of course, I'm sure that ICG will sell you some of theirs...

You can probably purchase something that will work just as well at Home Depot or KMart. Look in the housewares department. You will find many plastic storage boxes of all sizes and shapes that would probably suit your purpose.