+1 on what Ace is saying. Garbage is everywhere. That being said though it is possible to use knapping to turn a broken bottle into a more practical knife.

The flint knapping I've seen done (which is only a little) usually starts with a large piece and the person doing it would try to hit it in such a way that a piece would cleave off the right size for what he was making (in the examples I saw it was an Ulu style knife both times)...I imagine this takes a ton of experience and a little luck.

Though just to play devils advocate with myself, the natives of these lands of ours made a LOT of arrow heads in sizes as small as a dime. They were traded so much it was practically a form of currency. Maybe smashing the obsidian was a valid technique? Use what was there to make appropriately sized arrow heads? Somehow I don't think it was in their nature to be wasteful but then again I've been to Head Smashed in Buffalo Jump here in Alberta and that wasn't exactly the most frugal use of buffalo.