Cardboardboxes4U.com or just google insulated shipping container. There ae a lot out there. A little pricey but worth it if shipping some nicer stuff. Thats who i used when i used to ship a lot.
+1 I used to ship dwarf FW shrimp like this in winter and never had any issues. It's a lot cheaper than ordering some specialty insulated boxes and having them sent to your house...especially when you consider that you can just line a flat rate USPS box and call it good for less than $1 extra when you include the cost of the foam insulation board.
Thanks everyone for the help so far. I am actually trying to keep the box cold for shipping brine shrimp. If the temperatures are too high, they will use all the oxygen in the water and die.