Sentry utilizes an outside scheduler who may be offering shops for Nimble, perhaps. I have received emails from her regarding a few different MSCs.
I don't think there is a particular ladder to climb for the massage shops with Sentry. You just have to complete the training and they become available, as far as I know. Perhaps Dave can elaborate.
Most other companies that offer spa/massage products require significant experience with the MSC before getting assigned one, it seems. No offense to Sentry, but I think that the fact that client is a discount massage provider makes it a bit less of a 'plum' shop. It's also reimbursement only. They are not snatched up that quickly in my area and are self-assign.