Here's a very cool site for the hungry WW2 US reenactor - World War Two Ration Technologies. To be clear, this company doesn't sell the food, but rather the packaging and how-tos on how to make your own rations. It also offers WW1 era rations too.
The site itself is a plain no-frills design, but what it offers is great for any WW2 reenactor who doesn't mind putting a little bit of elbow grease into making their own stuff.
Note: The link is a little wiggy, so if it doesn't work, then do a search on World War Two Ration Technologies to get you to the proper place.