MRE meals have a lot of uses. They make for very easy eating when you're out camping or hunting, or they can be a central component of your emergency supplies. FEMA recommends every home or household have at least three days worth of food stored up in nonperishable states for each person in a home, and MREs are a simple and effective way to do it, as these field rations for active duty military personnel are intended to provide them the nutrition and energy they need to stay fit for duty when away from bases and camps and comforts of their kitchens, galleys, and mess halls.
1) Army/Navy surplus stores: This used to be the only place where you could find MRE meals. These sorts of businesses deal specifically in what their name implies. They buy surplus military supplies anywhere they can and then sell them to collectors and enthusiasts. Unfortunately, finding such a store might not be so easy anymore. The Internet has put a lot of such places out of business or driven down their volume and profits. You'll also likely have to live in a major city in order to find a local store, as rural areas just don't have enough consumer base for such a specialty store.
2) Sporting goods stores: Stores that focus on camping goods and hunting supplies have started putting MRE meals on their shelves. You're very unlikely to find military surplus MREs here, but you might find military grade options manufactured in the same production lines and created in alignment with the same standards. The number of options and supplies available might be limited to narrow shelf space, and you're certainly going to pay a premium for a brick and mortar retail outlet. However, when you need a few extra units for a last-minute trip, this can do in a pinch.
3) Online: The Internet is where you're going to find the greatest selection and prices, especially if you buy in bulk. Volume savings drive down per unit cost, and an order large enough for free shipping makes things even cheaper. Just make sure you do your homework about each retailer, considering that you want to make sure you're actually buying what you want, and not used surplus about to expire that's being presented as fresh stock. If you look on Amazon or eBay, you can find specific dealers with very reputable dealings, but read their feedback thoroughly to see how they do.
4) Catalogues: Even in the digital age, some outfits do all their business through a print catalog. Although, they might have an online catalog too. They do this because some rural survivalists and long-term hunters might get mail but not Internet.
5) Expos and conventions: These traveling shows bring together dealers, many of whom might run surplus stores, websites, or catalogs, and take them to many cities for weekend or one-day events where buying, selling, and trading happens. This can help fill in the void for cities or markets that might not have an Army/Navy store.
6) Active-duty personnel: Those still in uniform have access to buying many things through on-base commissaries, which sometimes carry MREs. They might be able to shop for you, if regulations permit.
7) Veterans: These are among the biggest enthusiasts of MREs, and many might have their own stockpiles they're willing to deal and trade from.
Now that you know 7 places to shop for MRE meals, you can stock up on them, creating your own supply, or enhancing, upgrading, and expanding your current stockpile. Just keep in mind to sort between military and civilian grades of MREs such as Eversafe or XMRE. Not everything listed as military grade is actually military surplus, but is made to military standards. That means you can expect a full meal in a complete kit. Civilian grade meals are more likely to have more variety in tastes and foods, but you'll have to check each kit to see what's in them, as they're not usually full meals like their military grade counterparts.
Another thing to look out for is of course the expiration dates. MREs are supposed to be designed to last for years in proper storage. Having said that, military grade units tend to last longer than civilian grade options. Of course if you're targeting actual military surplus, some of that time might have passed already. Just check your stockpile from time to time if you're accumulating for emergency purposes.