If someone is parting a car, that may be a good way to go, but will take some time to cut them out.
I just posted on how to cut them out (well, at leas some info on that) and this may be a relevant thread:
http://toyota-supra.info/forums/showthread.php?t=4474
I don't know how much shipping would be if you bought them from Champion Toyota, but I bet they will be the cheapest way to go minus the shipping cost.
Look them up at this site:
www.toyotaworld.com
That would be brand new and new parts (even with the great discounts they give) will be more than used most of the time.
By the way, why do you need quarter panels? An accident, or rust?
I do have rear quarter panel sections with the wheel wells that I want to sell.
I cut them out to weld in where I had some rust in my car (damn NY salty roads!)
PM me if interested.