Some I worked for were BARE, HS Service, Ipsos and Albatross International. I also did some projects for Auditor Service, but they are headquartered in Mexico City and I am not sure if they do projects outside of Latin America. BARE had the most work available. Albatross was for luxury brands. The surveys were the most detailed. That paid the best, but they had fewer projects.
I also used to do some store audits for a business called Mintel. I would have to go to stores and see how products were displayed, if the packaging was ripped, etc. Boring, but I needed the money.
Definitely don't pay anything up front. That's a scam. I found this article that could be useful:
Watch Out for Secret Shopper Scams | Take Care