1. We bought a mooring cover from Masthead -- they had a regular listing for it ten years ago, but I don't think they sell it any longer. Other local members have had trouble finding a non-custom manufacturer in recent years.
They're located near Johnson Boat Works, and I'll bet they still have measurements for the J-18.
(fair warning -- they make nice covers, but they are not cheap)
2. On a mooring, do you mean leaving your jib out in the sun without a cover over it? (or in the sun, but inside a jib cover?) If you don't already have one, I'd recommend getting a jib cover. I think they are pretty reasonably priced, but we decided to make our own (many years ago). That seems like it would be pretty easy for you to rig on a mooring -- it's hoisted using the spin halyard.
When our boat was stored with the mast up for weeks at a time, however, the sun would bake the jib inside the cover, so we took down the jib and replaced it with a wire (while on the trailer). Our wire was just the forestay removed from an old jib, so you could make your own version of that pretty easily. But swapping the Harken roller furler from the top of the jib to the wire and back was a pain every time we dropped the mast to put up the jib. I would not recommend doing that swap on a mooring. I think other owners have rigged more complicated systems for removing the jib while the mast is vertical, but I don't know the details of their setups.
3. There are still a few J-18s in the area, including two racing weekly on the Severn River, one in Arnold, and one at Washington Sailing Marina. There used to another one here as well, but I've lost track of who owns it now.
Fair winds,
Rick