A potentiometer with a ganged power switch is a pretty common item. AES carries a few:
http://www.tubesandmore.com/products...Potentiometers
You could also look at suppiers such as
http://www.mouser.com/,
http://www.alliedelec.com/ , or
http://www.digikey.com/ .
Mark Oppat also stocks controls, and he can even rebuild old ones:
http://www.oldradioparts.net/controls.html
Phil Nelson