A number of free resources are available for Trials of the Grail and more will be forthcoming.
I'm also very open to third party development of TotG material. If you write something for the game, and after reviewing it, I'll post it here.
I'm also adopting Ron Edwards' policy towards commercial add-ons: if you make it, you can sell it and keep the profit yourself. The only stipulation is that I get to review it (preferably you'll have been showing it to me throughout the design stages) and get to say whether it meets qualifications for release. Write me if you're interested in doing such a thing and we can talk about it.