There's Survey Merchandiser (app) - tends to be low paying but I believe you can negotiate with them a little.
SAS retail
SPAR
There aren't a lot that post here and some of them aren't "gig" type work, they might have recurring assignments at certain stores like a dollar store chain to do weekly visits to re-stock chips or dog food (I've gotten an offer for things like that).
There also used to be an association called NARMS, national association of retail merchandising, they dissolved and another association was created, I think it was WARES, world alliance of retail something. I don't know if they exist or not anymore but the merchandising companies have gone through some similar mergers and acquisitions as mystery shopping companies - some of the smaller ones have been bought out or closed their doors.
I would recommend trying to go on Indeed or LinkedIn and searching for merchandiser or merchandising gigs in your area, that is probably going to be your best bet.
Hope that helps!