I see you have been looking for a couple years. Attached is a link to a used equipment supplier that carries the negative carrier you are looking for. However they are out of stock at the moment but if you contact them and say you are looking for one they will go out and try to find one for you. The price is high but remember they are Canadian and US cash is worth at least 25% more.
Good luck http://store.khbphotografix.com/6x4.5-Negative-Carrier-for-Beseler-23C-Enlargers.html