Anonymous is not the way to go. I would flat out ask these little twits for the name / number of their manager. If you are being meek while they are treating you like garbage, then they will continue to do it. I am 33, but look like I am in my late teens / early 20s, and CONSTANTLY get poor service. I have found that when I am assertive and refuse to accept rude treatment, it often solves the problem. I am not above calling the employee on their behavior right then and there, even if other customers are present. Usually I say, "I'm sorry, is there someone else that can help me? You sound like you are having a bad day." Or "I'm sorry, did I call / come at a bad time?" This usually embarrases them enough that they back off.
I have the same situation with a certain employee at a well-known kitchen store. She treats me like garbage every time I come in there. I simply request to be served by someone else, or call her on it. Once, when I asked for a particular item, she said, "You mean the one that we NO LONGER CARRY?" I said, very patiently, in my teacher voice. "I don't know if you still carry them or not. I don't work here."
Good luck! This is one of my pet peeves!