The US military rations (Meals Ready to Eat or MRE) are technically US GOV property, so not legal to sell as i understand it. Although you will probably find them listed on Ebay from current/ex military, they will probably be listed as a 'collectible' or 'not for consumption' or something along those lines. There are also private sector versions of the MRE such as
http://www.cheaperthandirt.com/MRE125-1.html