Depends, do you want actual factory half doors, or aftermarket? There's a HUGE price difference.
Aftermarkets you can buy from all over the place online, from pretty cheap, to fairly pricey, and even used now (as per above post).
Factory are pricey, and then still need to be taken to a paint shop and sprayed but can be done. A company in the US sells a kit. I thought I had it bookmarked but I don't. If you REALLY want to look it up though head to JK Forum and wade through all the half door results under a search, someone posted the link before. Cause if you head to the Chrysler parts counter this is what you're looking at:
Topic: JK Half Doors (Read 1212 times)