I was poking around eBay looking for 15" aluminum cars for a project when I discovered that North Lima Train Works has a substantial, new presence there.
NLTW's username is "lioneltrainparts2009" and they list lots of diesel, steam, and electric locomotive shells, as well as passenger car shells, motors, etc. Want a replacement GG-1 shell, brand new? It's there for $50.
I know AA doesn't like links to the auction site, but use NLTW's user name in the eBay search function and you'll find plenty of new LIONEL parts at attractive prices.
Also, I did check a couple of items, and the eBay prices seem to be the same as those on their website.