You can see if your breed's National club has a Classifieds section on their website... that's always a good start. (Like with the National Mini Rex Rabbit Club "NMRRC", they havea chat forum plus a separate Classifieds section dedicated to Mini Rex.)
There's also another rabbit forum that is more "hard-core breeder" so to say (where certain topics are appropriate,but would not be so welcomed or appropriate here) - I'll PM that link to you if you'd like.
There are also SEVERAL Yahoo groups dedicated to Rabbit sales / classifieds and breed specific... http://dir.groups.yahoo.com/dir/1600042875 (I already did the search for you under "RABBIT")
This should give you at least a week's worth of a start...