There are several things that can help. First is, as you have pointed out, being registered with many MSPs. Second is hanging around forums because although our ICA keeps us from being specific in open forum of XYZ MSP shops ABC Client, on many occasions I know of this information being privately transmited. Third is to contact the client directly. I have sent emails or called the marketing department of several companies and said something like "I like being a customer of ABC and find some of your stores in my neighborhood are great and others sure need improvement. I am a mystery shopper and would like to know what company does your mystery shops for you so that I can offer them my services. Thanks ,,,,etc." I have gotten responses from EVERY SINGLE ONE of the companies I contacted. In one case it was the CEO who got back to me two weeks later. They have also been helpful in saying things like "XYZ will be doing shops for us 2nd and 4th quarter of this year." I have also gotten referred to outfits that do their employee training and mystery shopping and the outfit is neither a member of the MSPA, nor do they take on ICs.
Beyond that, there are many, many companies that seem to have seriously cut back or eliminated their MSing program for 2008. A response I got the other day was, "We hope to restart our MS programs in 2009."