@Marshall Fairley The wholesalers that get the most are Myers Home Buyers, New Western, and Net Worth Realty.
As far as a formula to use with them. Take the ARV - Repairs and you will be at their asking price. If you go with one of these three don't expect much room for profit if any.
Best way I know of is to do direct mail or door knocking. It will get you a pretty good response rate. I have found a deal out of about every 600 pieces I mail. Mail costs me on average about $0.50/piece (I use a mix of letters and postcards). $300 to find a deal is not bad. I think it is worth the investment.
While I agree with @Ryan Blake that Myers, Networth & New Western get the MOST houses, I would recommend researching other investors experiences with these companies prior to pulling the trigger. These companies do have deals, but I think its important to research, understand and know their motivations and roles in these transactions.
@Marshall Fairley just like @Ryan Blake suggested, the best way to find off market deals is probably mailing yourself. Wholesalers that leave meat in the bone for the flipper are very scarce these days. We have a branch wholesaling business but don't market in the DFW area. Try mailing a few hundred letters to motivated sellers in your area and see how that works for finding deals for yourself.
Good luck and happy investing!