You could buy them from a dealer, but it will cost you dearly. Keep checking ebay - someon is always putting aftermarket wheels on. Usually you can find a set of lightly used wheels/tires for < $1200 for the set.
Ebay is awesome. I built a full-size spare (for long cross-country trips) cheap. 1LT rim for $130 (inc. shipping) & $160 (inc. shipping) for the Goodyr HL tire (same stock tire) and $20 to mount at local shop. $310 *total* (no TPMS but you don't need it for a spare) vs what dealer quoted me ... about 550.. basically 600 w/ tax for ONE complete wheel (no joke).
I was laughing. Dealerships are funny. For $900 or $1200 you can get all 4 from ebay. Why anyone would I don't know but it's more to the point. I don't even know who to laugh at... the dealership as a whole or the individual mechanic, because I also called twice to get a quote on the individual wheel and the one guy said with confidence.. $320(-ish w/ labor) and another said "about $380" ... "ABOUT" .. I was like "wtf?" ... how can it be "about?" Like he just made up a price... took the $320...added labor.. and then added a personal Lunch fee so it will pay for his meal?
If the car comes with the "California" option, (20" rims, dual exhausts, Bose sound perhaps), does the speed limiter get set up to the rating of the 20" tires - which I think is 130-mph - up from the 2LT's 108-mph?