Where can someone pickup a few boxes for shipping AR15's or any rifles?  

7,try your local dealer, maybe they could help you out.

Instead of a box, you may want to get the cheapest Doskocil rifle case, which is the $14 #10486 I think. That way, you ship the rifle in a hard case, using the same box the rifle case came with. All you need to do is put the rifle in the case, the case in the same box it came in, tape it up, and wrap the lot in plain, brown butcher paper and you're all set. Now you have a foam-lined, hard shell case, which is much safer than a box with nothing but foam.

However, if you must use a simple box, the CMP ships all their Garands in foam-lined cardboard boxes. If you can find out where they get their boxes, that's a good second choice.