Cheapest route would be a junk yard drive shaft from any YJ will work and should not cost more than $20. Just make sure the slip shaft is in good condition, spend another $30 or so and toss some new joints at it and run it.
Did you check your local craigslist for jeeps being parted out? If all else fails, search your area for a drive shaft shop, suck it up and get a new shaft made or rebuild your current slip shaft.