KEH is one of my favorite resellers. But I've had good luck with all of the below. Another option to to check you local camera stores for classified. I got my RB67 Pro-S that way with 2 lenses, 1-120 back, 1-polaroid back, prism and waistlevel finders, and case for a very good price.
KEH in Atlanta www.keh.com
Midwest Photo www.mpex.com
Lens and Repro www.lensandrepro.com/USED/ubrand.html