Lowe's, Home Depot, Menard's, any local building supply company. Get the hand rail and cut it to length (if you get wood), and get 2 brackets that mount to the wall. If you have an inside wall, mount it on the inside wall so you do not risk installing on an outside wall and perhaps miss a stud, or even worse, screw through to the outside of the camper.
Depending upon you're camper configuration, you might be able to build supports from the floor up and then a attach the hand rail.
Another thought, check in the bathroom section and purchase a bathtub handicap hand rail. I installed one of those in my mother's bathtub a few months ago. There again, Lowe's, Home Depot, any home building supply store will have them.
