staySICK

The higher your level, the higher your chances are of getting one. From what I understand, they are generated when people craft badges. Not sure if one badge crafted generates only a single pack, though.

I've been getting them more frequently the past few weeks. Got one earlier today, actually. Think I'm sitting on 3 atm. Should probably get around to listing them on the market.

Once the global supply of cards for that game has depleted through people converting them to badges and such you can then unlock boosters next time you play.