Backs are not very expensive. KEH is a very reputable camera broker n is well trusted by most photographers here. Just Google them, you'll be very pleased with the quality of how they rate the equipment. A BGN tagged back will more than likely look new or gently used.
KEH also stands behind their equipment with a very good return policy.
Edit... I just checked KEH, they have a 120 BGN back for $21. Not a bad deal at all.
Thanks Paul. What I'm looking for is an ETR
C 120 film holder/insert. The ETR
S has a detachable back with a film holder/insert akin to a Hasselblad. That is Dad's problem. He has the ETR
C which does not have a detachable back and he just needs a 120 film holder/insert to replace the 220 film holder/insert he already has. I don't believe the 120 film holder/insert from an ETR
S back will work in the ETR
C. I did not see any ETR
C related equipment on the KEH website; however, maybe I just need help finding it.