A well-kept secret is that the online Disney Store has JC-related merchandise. No action figures, but t-shirts for adults and kids, mousepads, mugs, and a few other items. Good quality. Just go to the Disney Store and search "John Carter" and it will come up. Also, to support a sequel go to the forum johncartertwo.com. There's also an 8100+ facebook group, Take Me Back to Barsoom!
There is also a petition for a sequel to sign at http://www.change.org/petitions/john-lass
Just wanted to let you know in case you're interested! I think the main chance for a sequel is strong blu-ray and dvd sales, but that might happen over several years. Sales on Amazon seem to be pretty good, especially for the 3D/2D/DVD combo.