only promos made by members of the community. The Total War items, for example, were made by the same person who made the Bonk Boy set (Scout's soda super hero weapons and mask) and the Sleeping Dogs Cleaver. He was just a dude Sega liked his work and asked him to make promos and he gets paid every time a promo is bought from the TF2 store. The Sleeping Dogs promos were the same, made by members of the TF2 community, not Valve or Squeenix.
Other promos made by Valve or the company of the game are almost never sold in the store except in a few cases (Football Manager 2012, Quakecon and Hero Academy)
Those promos can be acquired now through random crafting of two regular hats and you have a 1/~300 chance of getting it. But because this system exists, you can also find people who will trade you the ones they crafted and didn't want.
It's kind of a shame they aren't in the Mann Co store, as I imagine a lot of people would like the ability to buy old promos and not have to mess around with trading. Just seems like wasted revenue potential for everyone involved (Valve and the company of the promo game)