Check out nationalbuttonsociety.org. It appears to cost $35/year to join, and I don;t know if they have a member's forum since you have to be a member to see a lot of the resources.

But, I've heard that local and national button shows are really fabulous and that the members are very knowledgable.

You can also check out Bead & Button Magazine's site. They currently have very little on buttons despite the name. Most of this mag has gone over to seed beading projects. But their "Button Corner" has 550 posts, even though the last was in 2012.