The closest source of reasonable (learning) quantities of knappable material is the Glass Buttes area in central Oregon. Checking your profile, that might not be easy for you to access so, given that our local agates and jaspers aren't readily available in quantity and size, glass is probable your best bet and garbage your best source. Tempered glass and 1/8 inch window panes are out, but anything else that is thicker than that will work pretty well. Broken mirrors, plate glass, bottle bottoms and old TV screen glass are good; anything thicker than, 3/16". TV tube glass comes with a few problems because it does contain some toxic materials so clean everything off that will come off before try to knapp it. The glass in TV tubes is leaded, but that isn't something to be concerned about other than adequate ventilation which you should always have anyway. Watch BOHUNTER's YouTube video on a safe way to break the tubes if you don't happen to find ones already broken:
http://www.youtube.com/user/BOHUNTER#p/u/13/NFm_94WSiGw. There is also thunder chert or Johnstone (old toilet bowl tanks) and porcelain of any kind that you could try but I would stick to glass at first because the quality of the various sources of porcelain, toilet tanks especially, is highly variable and you'll get really frustrated trying to learn to knapp on material that is just junk. Ideally, find a local knapper who will be able to get you started and will almost certainly have suitable materials to learn on. Where in Washington are you located?
