Years ago I spent some time doing mystery shopping. Just like most of the self-employment opportunities found on the Internet you need to wade through a lot of junk to get to the good stuff. When I first started I was pretty reluctant to give away my personal information online. I did extensive research on the various companies and opportunities and found a few which I really enjoyed doing work for.
There are many websites devoted to ways of becoming a mystery shopper. Some charge a fee for access to a database of companies and opportunities. It never made much sense to me to pay for information which could be found with a little research. I also felt strongly that I would not receive objective information by paying for it. Things might have changed in the years since but before you pay for mystery shopping information spend a little time on the search engines.
One thing I did pay for was a MSPA Shopper Certification. I believe it was $10 at the time and I found it totally worth the investment to get a better understanding of what I was potentially getting into. Also, the MSPA has a list of mystery shopping companies on their website. While I cannot vouch for the list or any of the companies it is nice to have the information all in one place.
Some of the assignments involved grocery shopping or visiting a bank. None of it was particularly difficult. Most required some sort of reimbursed out-of-pocket expense. This was especially true of restaurant shops. Be prepared to have to spend some money at first as it might take a few weeks for the payments to post.
I stopped mystery shopping a few months after I started. I suppose one of the major reasons is I just don't really enjoy shopping that much. I can say if you are looking for a fun way to make some extra cash, this can be the way to go.