Shadow Shopper is a 3rd party job board, that isn't directly affiliated with any MSCs. Schedulers from an MSC can use Shadow Shopper as an additional place to post jobs besides the MSC's own job board, in order to reach shoppers outside of the MSC's own database.
I personally use Shadow Shopper for shops that are tough to schedule.
It is "free" but the business model requires someone on either the shopper's end or the scheduler's end to pay to get the full shop details: Either the shopper can pay to become a "Gold" member, and they get access to all postings, or a scheduler can pay to contact the free "Bronze" members.
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Steven Cooley
Scheduler
Customer Impact, LLC.
Scooley@customerimpactinfo.com