There was a site selling stripped Ruger recievers for $75 I think but after ou figure in shipping and transfer fees (the reciever is the serial #ered part) you might as well just buy a complet gun and ebay off the unwanted parts.
of course you could go aftermarket. Volquartsen, and a few other manufcaturers do make and sell stripped recivers based on the 10/22 design. still, you'll pay more than a complete Ruger 10/22. All depends on what you want in the end.