20" Wheels and Tires

[Bill H]

I am thinking I need to invest in a set of 20" wheels for my 05 duramax CCSB, This is my goal to relaly get a max hook up at the track.
Question 1. Best place to buy wheels & Tires?
2. I want to stay if possible stock height what off
set, if necessary?
All pics of what you are running greatly appreciated...
Thanks
Bill