We used Valley Pig Pickin' (http://www.valleypigpickin.com/) for our wedding in Charles Town. They drove out with the pig roaster on a trailer and spent the day roasting and then served the pig and sides. Very good.
I know that there is a guy out of Charlottesville that does a pig roast every year for a friend of mine that lives in Winchester. Harpers Ferry isn't too far from Winchester. You might try caterers, event, & BBQ ads in the C-ville yellow pages