Or, if you really NEED some wither skeletons and can't find a fort, just make some Corrosive Gas with Emmasher's Gascraft. Step one: Natural Gas shale+Fracking Module+optional usage of Slickwater instead of normal water=Natural Gas in your system. Step 2: Refine 'dat shiet. BC Refinery will work(if you're a weirdo and want to use it) or the Engineer's Tool Box variant works as well, for half the cost(reminder, it can only work on single liquids, so great for some things, useless for others). Step 3: with your new found propellant, shove eet in a Mixer and add Gunpowder to the mix. Step 4: ???. Step 5: Flood the nearby Zombie Pigman village with said stuff, and watch 'em turn into skeletons and the occasional wither skeleton.
Bonus Step: Use said Corrosive Gas to build a Ender Pearl farm that has 100% efficiency as every enderman exposed to said gas dies instantly and has a 100% chance to drop his pearl.