I'm assuming front range?
2 main choices, 2 alternative choices, and options.
SuperPass: Copper/WinterPark, info at the passwagon site above
Colorado Pass (or 5 Mountain Pass): Breck, Keystone, ABasin, 10 days at Vail/Beaver creek http://www.snow.com/passsales/
Options also include 4 packs to individual resorts.
As for where to buy, yeah, it was said above, wait till September and it'll be hard to miss. As for which to buy, that is way too difficult to answer without more info on what exactly you are looking for, and often comes down to personal preference for certain mountains.