These collets show up on e-bay occasionally. Be prepared to pay more for the collet than most folks sell their RAS for. I have seen prices in the $50-$75 range for just the collet.
I have my Dad's old Montgomery Ward RAS and the collet thread is 7/16 X 20. My saw has 2 spindles, one for the router collet and one for a drill chuck.
Good luck in your search,
Basically, a tool is an object that enables you take advantage of the laws of physics and mechanics in such a way that you can seriously injure yourself - Dave Barry