Major Surplus & Survival sometimes has random bulk boxes at a reasonable price. They also have a good price on millenium bars, if you're interested. Emergency Essentials is another good site to comparison shop (they have good emergency articles too). Most places that sell MRE's include a shelf life table on their website, because shelf life is dependent on temperature control. I'm sure others on here have their favorite sites/places to shop as well, and can give you more options.
This is a pretty good cookbook, Backpack Gourmet, for homemade "mountain house type" dehydrated meals. The only thing I don't like about it is they take up way too much space talking about simple beverage powders.
I don't like the taste of most MRE's, so I try to stick with home canned/dehydrated foodstuffs mostly, but when I see a good deal I'll pick up some MRE snacks, Mountain House foods, millenium bars, etc. Just for variety and convenience.
I occasionally buy the MRE Entrees from The Sportsman's Guide - cheaper than full MRE's and you can add all the froofroo a lot cheaper anyways.
For camping, I prefer the "Ready To Eat" rice packets - Uncle Ben's, Zatarains and Rice-a-Roni all make several flavors each. I just ate one recently dated "Best by March 2007" - tasted okay, and I didn't get sick. But they normally are good for a year, or year and a half. I have had a couple go bad BEFORE their use by date. Add a meat packet (tuna, chicken, SPAM, etc) to it, and it makes a meal! I carry them on all-day hunting trips too.