Regular free Jobslinger.com is a quite good resource to find what companies have shops in your area. Things to be aware of: It pulls primarily from SASSIE reporting platform companies but also pulls from the MSPA job board and the Volition.com job board. It does not pull from companies that use the Prophet4, ShopMetrics or other reporting platforms or from proprietary platforms UNLESS those shops are also posted on the job boards at the MSPA and Volition.com. Thus it will be only a partial picture of shops in your area. Also the jobs you see posted are very likely to have been taken already by other shoppers, so the real value to you is to know what companies to sign up with for the future. As a general rule clients stay with the same company at least a year so if you see your favorite store was shopped in January by ABC, the chances are good they will shop it all year long. Shops vary in how often they are performed. I have seen some that are done twice a week all the way to one-shot shops. The most frequent intervals are weekly or monthly.
Some folks swear by JobslingerPlus as it allows them to make changes to their profile at all SASSIE companies with one data entry if I understand correctly. Some shoppers use their smart phones to receive shop emails and notifications during all of their waking hours. JobslingerPlus they feel gives them an edge as they will get instant notification when SASSIE shops post. This may or may not be important to you. Cost of Plus is I think $5 per month, but I'm sure someone who uses it will pop on here and tell you.
ICPro was touted as identifying those shoppers who are "Professional" as opposed to the rest of us rabble out here who only do this for years. It is free but I have never seen a need for a middleman and have to laugh that every newbie shopper can register as a "Professional" with ICPro whether they know what an ICA is or not.
Legitimate? Yes, probably. Useful? Possibly. A better way? Not likely.
Frankly there is nothing that is going to replace getting registered with individual companies, attempting to get your first jobs with each company that is offering something in your area (with the exception of those companies that folks are talking about needing to chase payments or not getting paid for accepted work), knocking their socks off with the quality of your shops and reports, evaluating for yourself after you do a job or two for them whether you feel they pay fairly for their expectations and are overall comfortable to work with. 'Go the extra mile' with the companies you decide to nurture in your stable and build an excellent reputation with them.