We have this one...
http://www.buybuttonparts.com/2-1-4-...ton-Maker.html
and it works very well (although it jams when someone who should know better inadvertently puts two of the dome-shaped pieces in at one time, and is very hard to open back up - so don't do that!)
I know some teams get their buttons made for them, but our team really enjoys designing buttons on the fly and even making them at the competitions...it is a way to keep everyone constructively busy when one or two of your "sub-teams" have something to do. Last year we started "customizing" a simple pattern we made at the competition, and as a result everyone wanted their own special design...it was quite a hit.