Free: Contests & Raffles.
My ultimate goal is to knap a blue agate into an arrowhead. One showed up on here a couple months ago. Holy cow that thing was awesome. I've been using some agatized/opalized wood. It is extremely unforgiving, so good to learn on I guess. It was free. When I picked up a piece of obsidian, it seemed ten times easier to work with. Both materials create nice bloodflow without notice. I would think chert is the easiest to knap. I've tried some jasper and other materials just because I had it.
I've dabbled KC. Not very good at it yet. Carpsniper has turned out some gorgeous stuff.
He had a few posts two or three years back that had some beautiful knifes he had made.