I had the same thing happen to my 5th wheel. I can assure you, when one goes, they will all go. Full replacement is the only cure for your problem. In my case it was a living room slide, a kitchen slide, and a bedroom slide. All three seals were replaced without the need to remove the slide. However, in the case of my kitchen slide, it did require the expertise of someone less than half my size. The kitchen slide is 16' feet long, and because of the flange arrangement on the inside it took a thin, limber person to crawl up into that space to affix the new seal.
As for the place to get a seal; my guess is that your seal is like mine in that it is a custom design for your rig. I got mine direct from the factory. Maybe Jayco uses a generic type, but I would ask them directly.