If you have time, money and the willingness to travel the world to learn cob building, then Cobworkshops.org can point you to a variety of workshops. They specialize in cob and do not list workshops related to other styles of natural building.
Begun by Coenraad Rogmans and James Thomson of House Alive! Natural Building and now maintained by Michael Blaha of I Love Cob, the site currently lists events on both coasts of the US and Canada, Mali in west Africa, Hawaii and France.
The site also has links to a number of cob building related sites and to news items related to natural building.