There's always Dragon*Con in Atlanta, GA. It's not technically a LOTR convention, but it is the biggest SciFi/Fantasy convention in the US. Not sure how much LOTR-related things will be going on there this year, right now the only actor from the Jackson movies that's scheduled to be there is Brad Dourif (Grima). There may be more added later, I'm not sure. In any case, just a suggestion. For more information, visit
http://www.dragoncon.com.