If I'm making a 6 inch cake do I get the 6 inch box or seven? even if using a 1/2inch thick foam board? is this the same for other sizes? anyone know of a site I get get the really thick sheet (1/2 and full) cake boxes from that don't only sell in bulk? Thanks.

It depends on what size board you are putting it on. For a 6" cake I use an 8" board and 8" box. You should be able to get boxes at Michaels or other craft stores. Most of them aren't thick though.

Brpboxshop.com sells boxes in lots of 10 or 100. I like the quality a lot. Prices are significantly cheaper in lots of 100 but most of us don't go through that number of boxes.

Most of us prefer a board 2" larger than the cake thus a 6" cake would be put on an 8" board and into an 8" box.
As for thicker boards use two or even 3 for your larger cakes . And if you can turn one of them around so the corrigation 'stripes' form an x or +..... that gives even more strength. It's really not possible with rectangle boards (1/4s, 1/2s or full).