North American Land Sailling Association
This Forum was set up by John Eisenlohr. It is a yahoo groups based site. You have to join to participate, but you can view postings without joining and can join without a Yahoo address.
Sea Breze is an Australian sail sports forum that has many landsailing threads running, including a great thread on landsailer construction.
FiSLY is the international organization that regulates and promotes landsailing. They have forums on a number of landsailing-related topics
The Landsailing Facebook page is probably the most active landsailing discussion page going in the US.