I'm just going to say, a lot of people won't give you information if they think you're in their area, because unlike a regular job mystery shopping is a competition, the ones who have been around for a long time have established relationships with companies and schedulers and they will be offered jobs sometimes before they get on the board. You have to build your own relationships and don't expect to bring in a lot of money, especially in the beginning. I've been doing this for more than 10 years, the companies I work with may not work for you. The more experience you get, the more you'll know what you want to do.
Now with that being said, I will give you some free advice.
Keep meticulous records. Start with a google sheet. My sheets are monthly they have companies, locations, addresses, mileage, fee's, tolls/expenses, reimbursements, a section to check off when paid and keep everything you need to prove you were at that location, at that time, for that payment for the IRS. Especially if you have a lot of expenses and mileage. My CPA said better safe than sorry.
Print off your sheet that has the information for that assignment and make sure the fee is correct, just incase there is an issue later and also keep emails for big assignments when you negotiate higher than offered fee's, make sure you hold onto them, until after you're paid. Sometimes mistakes are made and you should always have a paper trail.
Read all of the guidelines, for every assignment, because they do change from month to month and if you are doing photo audits for gas stations, take extra photo's and save yourself a trip, because they will send you back out at YOUR expense or you won't get paid. While you're working assume someone will be checking your work. Some shops put in their guidelines that they will be reviewing your visit, so make sure you follow them. Don't be afraid to ask questions, most schedulers will answer them.
Good Luck! and as for companies I would start with the Presto app and get some experience.