D.M.
G.,
There are several companies who hire legit Secret Shoppers, however they have very stringant requirements and can be very competitive. I used work for an amusement park that hired their own Shoppers, and while it paid well, they didn't get a lot of hours and it was very much an on-call kind of thing.
A friend of mine's Mom who has hip issues, got a job working at the local hospital in the gift shop and she only works a few days a week for a few hours.
This link is a great resource for finding out about disability and training...you can call your local agency and see what is out there.
http://www.edd.ca.gov/Disability/
Good Luck.