Check out "Dyeing and Bleaching Natural Fly-tying Materials" by A.K. Best. He has it reduced to a science--everything I have tried in it has worked great. It has some good recipes for rabbit fur dubbing blends as well. Good luck!
Yard sales are a great place to score loads of dubbing material as there is usually a bag of yarn for a buck somewhere in the mess. Knitting shops are great resources as well as they have loads of mohair, wool and poly yarns as well as some chenilles.
Take yarn of your choice and cut it into pieces varying from 1/8 to 3/4 inches in length. Place them on a mixing card and start combing te material with another card or comb to seperate the material's individual fibers. For fine dubbing you want to start with soft yarn that is cut into smallish pieces. For rough dubbing, as in a seal substitute, start with a coarse material and keep the fibers longer (3/4" or evn a full inch).
For blends, keep track of recipes based on inches and sizes of pieces for easy duplication later on. An index card per blend works great.
6" of black Aunt Lydia's, 3/4" hanks
1.5" of royal blue poly, 3/4" hanks
1.5" of red poly, 3/4" hanks
1.5" of evergreen wool, 1/2" hanks
3" of pearl Flashabou, 1/2" hanks
I also concoct what I call Deer-Hare dubbing blends. It is the hair cut from 4 square inches of deer hide blended with fur cut from 1 square inch of rabbit hide. Blend these together using mixing cards and you are left with a very spiky dubbing. To see how I work this, goto the following:
I just bought an old house that had many cats. I stuck my head in the duct work last weekend and it's stacked in hair that looks like it'd be a good substitute for hare's ear and the like. Let me know if you'd like to come over and grab some. It would help with my fierce alergy to cat hair.
Goof advice above. Here may be the most basic and elementary blending skill: Take two, or more, loose little bunches of dubbing. Put them together between your fingers. Start pulling the bunch apart; put the pieces back together, positioned differently. Repeat about two dozen times. This will give you a fast, workable blend; perhaps not as perfect as machine-blended, but effective.