Criminal Law Questions? Ask a Criminal Lawyer.
the best guidance i can offer is for you to contact the corporate headquarters of each store where you believe this is occuring - it should be written communication mailed certified with a return receipt requested wherein you will explain what is going on and ask them to assist with investigating the matter.
a private investigator might be able to help you i am not familiar with the surveillance personal company but you can can them and see what they offer. the thing is the stores do not have turn over anything to anyone absent a subpoena and you cannot get that unless there is a lawsuit and you cannot file a lawsuit without knowing who did this. so you might consider the PI to start or contact the corporates. believe me, they are never too busy if they feel one of their stores might be a lawsuit target
if you feel certain you know who it is skip the PI and move on to the next step.
i suggested the letter so you could try to handle this yourself; however, if you are seeking legal counsel let them write the letter as it will have a greater impact.