Silver and Gold certifications are available on the MSPA website. They also have a number of other courses including:
I wouldn't suggest paying for any courses to mystery shop.... Everything you need can be found through free courses - often found on the websites for various MSC's or other forums like this one. If you want to know how to do something better than you are now, I would suggest simply opening a thread and asking the question. You are certain to get a few answers.