I looked on ebay. Does it have to be a unidrum, not a beseler or some other? I'm curious how you plan to get 4x5 negatives secured in an 8x10 drum. You're going to develop your BxW 4x5 negatives in a color print drum? I'd like to hear more on this.
If you find one without the neg separators, you can use small rings made out of PEX (flexible plumbing tube). Cut a ring about 1/4" wide with a coping saw, and then make a split in one side. The PEX has some nice flex and will fit over the V channel on the inside of the drum.