Keep checking eBay in the "TV tubes and parts" category -
http://www.ebay.com/sch/Parts-Tubes-...from=R40&_nkw=
Just type-in "knobs" in the search. Eventually what you need will come up.
The GE "locomotive" is one of those "must have" sets in any vintage TV collection IMO. My example is one of the best working vintage TV I've ever owned.