You would probably have to build one or have one built. Its not that hard to do. You basically need to have specialized jib made that will be short on the foot and have a high clew. You need to construct a jib boom which is easy to do with piece of aluminum tubing. The boom needs an outhaul and a gooseneck fitting but both can be very simple. You can rig a self tacking jib with as simple a sheet as a centerline tackle but more typically there is a horse or traveler for the sheet tackle. The sheet then runs forward to the stem and then back to the cockpit.
Much of this depends on the particular design of the boat but a self-tacking jib will greatly reduce performance compared to the typical ''lapper'' used on most small boats. You would be very far ahead by simply improving your sheet handling gear. The simpliest improvement is to put a 2:1 sheet on the jib and a set of ratchet blocks. On the typical 18 footer a jib is so small that this will actually be easier to trim than a self tacking jib which has a lot of friction.
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