Craigslist works great. The first "extra" pigs I sold were to people I worked with. Having a couple pigs came up in conversation, and people were kind of intrigued. It was a lot easier than I would have thought. Since you're not getting them until spring, talk it up a little bit and you may have the extra one (or two) spoken for before you even buy them.
I had a butcher who swore by a 1/4 - 3/4 rule. Selling 3 halves will make the half you keep free, processing included. It's pretty accurate, if not even a little conservative.