I hope it is something other than ringworm, but ringworm will "go away" in time, too. So, since you said it disappears at times, it could be. In the mean time, providone is cheaper than betadine. Scubby, washy, rinse and spray a bit on. Buy bleach and use it on your brushes, whatnot. These measures are not expensive and it will not adversely affect anything. Perhaps you could talk to your vet about collecting a scraping yourself. I am sure clear direction are on line (it isn't that hard). It would be a bit hard to believe that a reasonabley well obtained scraping would be accidently contaminated w ringworm or rainrot microbes, even if it were not taken by a "pro". That way you would only pay for the lab work.
Add on: and, you know that red lysol concentrate that really stinks?? It is an amazingly cheap and effective dissinfectant, too. You have to dilute the heck out of it. It would be and alternative for your brushes, whatnot. Bleach has the nasty side effect of "bleaching" one's clothes. :)