They didn't have it in Fleet Farm?? I got mine there, it was in the aisle with all the large animal dewormers and such.
Fiporil is a broad spectrum med too, but either should do the trick.
They aren't necessarily ear mites, but they do attack the ears and the skin. Your hen may have flaky white dead skin all over in that area too. But you really should try to get that gunk out, it is like leaving pus in a wound. You want to take a finger and thumb and start about a half an inch on either side and squeeze up like you are popping a zit. Trust me, I know it is gross, but now that you have critters, you should probably get yourself accustomed to doing some things that are just gross, it comes with the territory