Roughly how much would it cost for parts, to fix a G35 coupe that needs a new rear bumper and rear quarter pannel? Havent seen an aftermarket rear bumper that looks close to stock (size)

 

keep your eye on the private classifieds here on driver. Typically used oem rear bumpers go for very reasonable prices. shipping is where you can drop some coin, but if it's already painted your color it can be a great deal.
I've yet to see a quarter panel, your best bet would be a salvage yard or parts counter at a infiniti dealership (if you have the scratch).

 

Rear bumper is easy, check your directory for a Nissan/Infiniti wrecking yard. The quarter pannel is a whole other issue, replacing one requires a qualified body shop....it's won't be cheap!
Gary