My favorite bottle bottoms are from the 22oz. Guiness Stout bottles (they're a bit thicker & flatter than other similar size bottles I've tried). I do the initial breaking with the bottle inside of a plastic bag to keep shards from flying everywhere (I just hit the bottle in the shoulder area with a hammer). I trim off any remaining pieces of the side with a quartzite hammerstone (stone picked up from a nearby stream, although I suspect just about any rock would work). Most of the home improvement stores sell copper ground wire by the foot; buy some of that and a 1" dowel and you're in business as far as pressure flakers goes.
Will