An original RCA theremin was listed for sale on eBay today, complete with an RCA speaker and a colletion of spare tubes! This one appears to be in decent physical condition. The listing says it "Works well and sounds very nice".
The auction is set to end on September 27th and has a current bid of $1000 at the time of this posting.
Of course, if you already have enough RCA theremins, why not look for other theremin bargains on our theremin auction page.