You should be able to find it at an RV dealership, methinks. A couple of years ago, I got a length of it from a small RV dealer near where we kept our rig during the summer. If you get it, get some butyl (putty) tape to put under it when you mount it.
Used mine to prevent rain/snow melt from running down the sides and into the replacement cargo doors after the original ones swelled due to water being absorbed into them from the frames.