TrimLoc sells direct but only in huge rolls. But there are several ebay sellers who have this stuff by the foot. I don't remember the size but just measure yours and then look it up on ebay. Be sure to specify the correct color.
If you can locate a gentleman named Tom Grieb on the "U-Haul Camper" Fb page he has the exact size and model number and everything. I think this is it http://fiberglasstraveltrailersrv.com/Vinyl-Trim-Lock-White-1-2-H-x-3-8-W-U-Haul-Burro-Campers-1350W1-2x3-8.htm?categoryId=-1