This year, going from ND to WI for more bird hunting, I had a cooler with my Pheasant possession limit in the back and needed them to stay frozen for 5 more days. A quick search located a dry ice retailer in Fargo, ND. 30# this time kept them frozen from Tuesday until Saturday. The guy in Fargo was only a mile off the interstate and didn't slow me down much.
It isn't that cheap- about $1/lb., but I think it's worth it to keep the catch fresh. They wrap it in heavy brown paper and give you a warning about breathing it (for the moron). Definitely worth the extra effort.