H'ok - here's my spiel, mostly addressed already:
Milsurp ammo: Mostly corrosive due to salts in the primers. Cases usually laquered steel and wrong priming system, so not reloadable. Countries that are selling surplus usually sell their oldest surplus each year, so each year more surplus is available. Vary the country of origin, and the sheer amount they're selling each year, and it's actually relatively common. The greater issue is importing it, as one of our ministries has a hard on for mercury primers (mercury is used in making them, in incredibly small amounts). Therefore, the ministry bans them from coming into canada, yes lets oil and mine tailings pollute groundwater sources.... you get the picture.
Milsurp 7.62x39: On the west coast, rediculously common. There are several stores that have more crates of x39 than guns, and each crate is usually 1120 rds.
Cleaning: Many people use ammonia/windex. Ammonia dissolves salts, but is caustic, expensive, and smelly. What many people do is in fact give their gun a bath. The cz is especially suited for this because there are no parts on it that can't get wet. To do so, simply boil a pot of water, then pour boiling water on all parts while scrubbing with a brush. Submerging easily removeable parts in a plugged sink also works. This way the salts are dissolved by the water, and your part is heated up, and will self dry by evaporating the remaining water. Continue cleaning and oiling as usual.
Gas system: During each shot, a little of the gas propelling the bullet down the barrel is redirected out a vent in the top or bottom of the barrel. That gas pushes on a piston/rod, that literally kicks the bolt, which then flies backwards and cycles the action. Your PAL book has pics.
Availability: any major area can get x39. Commercial is usually $15-$20/box at a reasonable price (I buy igman for $15/20), but it varies on maker and store. Commercial can be reloaded, so save your brass to either eventually reload or sell to someone who does.