Well I was trying to find the answer to this too. Beseler doesn't make them anymore, hey won't even give me an old part # for them. I asked a seller on eBay what his thoughts were since B&H doesn't sell them anymore and his was his reply:
"I took the carrier apart and measured the o-ring and best I can come up with is it is about 0.10 in diameter and roughly 32 inches long. When I do my rough math that tells me it is about 10 inches in diameter (PLEASE correct me if I'm wrong). There is an industrial supply house that makes up o-rings for hard to find sizes. I punched in the sizes and got a price of $8.93. I'm attaching a screen capture of the info. If it doesn't come through, the company is McMaster and you can find them online."
Im hoping to find the 120 negatrans and if I have to order the O-Ring from McMaster I will let you know if it works.