Wrong forum - but what the heck...

http://www.gunbroker.com
http://www.ram-line.com/rifle.aspx
http://www.rugerstocks.com/
http://www.brownells.com/aspx/ns/store/catsearch.aspx?c=3106&p=4816
Brownells can get one if not ininventory

I don't think chamber spec matter for the stock over the model. You can also but cheap m77 in gunbroker and take hte stock - I see them here and there - some go cheap, some not.