Mile High Shooting and Euro Optics usually have lots of AI chassis systems in stock. And they are site supporters here.
We keep AICS and AX chassis systems in stock, mostly for our builds but we sometimes have extras. I know other builders do too. If you have a favorite 'smith, you might try contacting them to see if they have something. I bet Greg Young at Bugholes has something too. If you want a quality stock with a really good aluminum bedding block other than a chassis, John Whidden's stocks are outstanding.
Mark