Ear mite medication is over the counter stuff. Try taking some of the wax and placing it on a black piece of paper. The ear mites or white and look like tiny spec's, if you look close you can see them moving. They are very tiny.
Try one mite-regime...cats hate it, so if it doesn't work, you don't want to repeat it.
Can I get this over the counter?
I find it hard to believe your vet is a total idiot...if he says it's not ear mites it's possible...I'd dose him just in case.
I had a dog who had this problem. It was finally figured out that it was caused by him sleeping on that side all the time, and the heat/moisture build up was causing it. So...I can't tell you how irritating it is to poke your dog every 5 minutes to get him to roll over onto his other side. It's not very practical...