I'd like to include a custom FB share button right in the middle of one of my posts. (in addition to the built in like button at the bottom of the page). I can't get the http://www.facebook.com/sharer.php link to work, guessing no php in the body which makes sense. Is there another way to make this happen?
You'll want to use code direct from Facebook to do this.
Here is the information on how to add Facebook (https://developers.facebook.com/docs/plugins/share-button). Additionally, if you're interested in using Twitter, here are their instructions (https://support.twitter.com/articles/231474) on how to embed a share link.
Complete and current information on how to use NationBuilder is available in the documentation section.