Green keys are a dealer item.... anywhere from $17 to $25 each, or you can get them from
www.gmdieseltech.com for $55. They are stock half-ton keys. Stock 2500HD keys are purple.
Green keys are NOT the proper way to lift your truck. If you have a 4wd, they will not only pinch your ball joints and tie rods, but will also put your CV joints at an extreme angle. For a street-queen that is not used offroad, they're "okay" (not good, just okay). For any kind of halfway serious offroad use, you'll be replacing CV joints all the time.
Putting bigger tires on will further increase the stress on your ball joints and tie rods.
To top it off, your ride will suck.
Just pony up the cash and buy a "proper" 6" suspension lift.
You should be able to get 1-2" of lift by cranking your stock keys. Any more than that and you're asking for the above problems.
I'm running 285/75-16 on 16x8 wheels with just a little trimming on the driver's inner fender well and the bars cranked to level the truck... all four fender lips are at 41".
Biggest thing to watch on aftermarket wheels is backspacing. For an 8" wheel, you need AT LEAST 4.5" of backspacing or you'll have serious clearance problems with 285's or anything larger. Stock wheels are 6.5" with 5" backspacing.
I'm pretty sure that any 10" wheel is going to need more than a TR crank or green keys.