Odds of Getting In
There's plenty of work out there and plenty of legit mystery shopper companies who will hire you. There's also lots of scammers claiming you will "get rich" from mystery shopping. If anyone tells you that you'll make a mint off of this job, or asks you for a fee to DO the job, then you know they're not legit. Remember, they need to be paying you, not the other way around.
The Mystery Shopping Provider's Association lists dozens of companies eager to pay you less than minimum wage for your thoughtful, honest prose (source).
With a plethora of options, your odds are great.