Items can be found in-game based on the time spent in-game. Currently there is no way to influence which items you find; it is completely random. There is a limit to the number of items you can find per day, and the longer you spend not playing, the more frequently you'll find items for a while once you come back to the game.
Weapons like the Blutsauger can be found via achievement also for a couple reasons. Valve set a precedent with that because the Medic pack and some others were released before the random drop system, and that was the only way to get the items at that time. It is also because competitive players need access to all the items so that they can be on equal footing with each other, so there needs to be a way to reliably get a specific item. At least, that was one of the reasons they added achievement-items back in the Sniper/Spy patch. I don't believe they still think this, with some of the new items not added through large class patches.