Hey there! Welcome to the forums! Those storage tubes are sold at RV places, including Camping World. E-Z Hose Carrier - Product - Camping World
I'd check with one near you to see of you can get the cap you need. If they don't have parts in sotck they may be able to order you one from their supplier. It's worth asking. (If it's like ours, it is a twist & lock cap that fits into the end of the pipe in a groove and turns to lock.) Good luck!