I'm fairly sure they don't sell them like that, but if they did I'd imagine shipping would be an arm and a leg. It's really not that hard to do, but if your set on just replacing the board I guess a boneyard would be your best bet.
'98 xj: 3.5" RE, 31's, nates front bumper, etc etc...
'90 xj: Wrecked, waiting for the day it can become a full blown trail rig.
I redid my '84 headliner last fall and it turned out great. I did try the fiberglass resin trick but decided that it would run about $100 for enough goop to cover and seal the fiberglass soo.. I went to Wally Wally world and picked up about 10 cans of their .94c spraypaint and hosed down the fiberglass side of the headliner. Let it dry in the sun for a few days THEN applied the 3M adhesive and a new headliner. Worked great. Kept the glue from really absorbing into the 'glass. Use 2 cans of paint at a time. 1 in each hand works great and is kinda fun. Do NOT cut the headliner fabric too short of you will be back for another peice. Roll the ends over the 'glass and glue.
X2 on doing it yourself. You can't imagine how easy it really is. I went to a local fabric store and picked up some fabric, went to Auto Zone and got some 3M spray adhesive. Took the old headliner off, scraped off the old foam, sprayed it spread out the new fabric, let it dry for a few hours and reinstalled. Took less than a day to do, cost around 50 or 75 bucks, and looks TONS better!!! On a scale from 1 to 5 I would rate it a 3 ONLY because you have to take all the trim panels out of the XJ to get the headliner out. Other than that it was easy as pie!
I recently redid my headliner in my 92. Most any fabric will work (REALLY digging the camo headliner). Most specialty fabric stores will have the actual automotive headliner if thats your style. Not very expensive either. I used the adhesive spray from Advance Auto. Comes in an orange and blue can I believe. Never tried the 3M stuff.