I can't speak for everywhere, and it's been a long time since I was authorized to shop on-base, but back when I was, the 550 cord they sold at the X was different from the stuff the pallet jockeys over in the cargo rigging shop, and the folks in aircraft fitting used to build the actual parachutes with. The X is basically a civilian organization that has a contract to sell stuff on base. They sourced their 550 cord from the commercial market just like anybody else. The stuff that the chute shops used is acquired by the DOD's purchasing arm and is required to meet their specific requirements for official use by the military.
Here's a link to the actual mil spec http://www.takknife....ves_g000017.pdf
If you bother to wade through all the militariese, it says that the cord has to have a identifying color pattern on one of the internal strands that identifies the manufacturer, and that the cable must be inspected in a specific manner for each lot of a given size. This adds a pretty penny to the price, but does not in and of itself, make the product one bit better than the commercial versions you can buy just about anywhere. It does make it consistent, which is what I would want if I had to jump out of an airplane hanging from the stuff.
Out there in the civilian market, there is 550 cord available that is just as good or better than the mil spec version, but there is an increasing amount of cheap crap that is not even worth braiding into hatbands or whatever. Caveat emptor!
Personally I buy most of mine at my local command post surplus store, where I can open the packages and fondle the merchandise before I pay for it.