I think you can buy a fishing rod from Myaun if you are locked out from getting it from "Outside the Law" quest (not sure tho haven't tested it but I bought one while I was doing the quest). As for the trout, you can get it by fishing at the river.
Hmm...weird. He is definitely selling one in my save. And I can't think of any other way of getting one except on doing the "Outside the Law" quest and having Rohan make one for you. You might need to start a new save and don't burn the forest.
