Not cheap, but at least available at B&H Photo.
http://www.bhphotovideo.com/c/search...tialSearch=yes

Choose the ones marked for, “LPL/Saunders 4x5" and Omega D5 Enlargers”. They don’t specifically say 4550, but they fit. I have an LPL 4550VCCE XLG with those three sizes.

John Powers