What kind of RCA set is it going on? If it's a set from the 50's or 60's, you might have better luck finding one here. For instance, if it's a color chassis from the 60's or early 70's, some of us have spare chassis for parts. An NOS RCA unit might be difficult to find for those years.
NOS units for the more modern sets would likely be easy to find... perhaps at Moyer's Electronics at
http://www.moyerelectronics.com/ If you can't find the part on their website, be sure to call them on the phone and give them a part number. They might find it that way.
Do you know the model number of the set, part number of the xformer, or a cross reference number? Roughly what year is the set from?
Details... give us details!
