What is your location? I may have one or two, used but good. CTC18X-CTC201 (and possibly the ATC101/M134 chassis) all used the same part number thermistor per RCA. If you are in the US, PM me your address and I'll get one to you. I junked a curb-crawl RCA CTC203 in 2010, and another RCA not-sure-of-chassis in 2012.
Of note - most BPC sets of late (as WA2ISE correctly pointed out), use a PTC thermistor, usually 15-25 ohms. I had a stock of RCA 141239 thermistors I used or gave away, and they replaced about 90% of the 2 legged kind in 32" and smaller sets.
I scavenged the 3 legged TDK/Kamko thermistors from anything I junked, so I had plenty of both kinds at one time. I may still have one or two, but just about any 2-legged thermistor from a BPC set will replace any other. For a 36" set such as this RCA, the inductance/resistance of the coil places a bigger load on the thermistor, so you'll want to have the original type if possible for longer thermistor life.
Old sets using the thermistor/varistor combination used an NTC thermistor, and most are replaceable with a Workman FR191, FR291, or FRTV kit. Oneida, Ambassador, and GC Walsco had similar thermistors with similar part numbers (GB191, GBTV, etc.), typically 100 - 150 ohms cold.
Just please - don't go buying a 25 dollar thermistor on eBay - what a ripoff. My CTC16 has a new Workman I got off of eBay for 2.99, plus 4 bucks shipping. ......and then I found my stash of thermistors.
Cheers,