I have been building my own for a while on the table saw however, since i havent had much time lately, we have been buying them from our wholesalers. If your LFS cant help, let me know and i can sell you a few (not for profit).
More suggestions:
1. You can buy 3/4" thick styro at home depot, buy the cardboard boxes at OfficeDepot and just cut the styro to fit the box with a razor blade. This is time comsuming but the cheapest. you need to be accurate with your cuts or else the styro will be loose at the corners and your box wont insulate well.
2. Ask your local pharmacy or lab if they are throwing away any specimen boxes. Pharmacies and labs often send and receive merchandise inside insulated boxes.
3. Im going to look for a link, if you do end up using it, please let me know how well you like it.
JOHNNNY