dweste,
I took a class last summer for archeaological field work, one of the things we did was called 'experimental archaeology.' We learned how to find the right material for stone tool use and a very basic way of reading the ridges of the stone and how to hit along the ridge to get a good basic core.
In that session of the class I never got beyond getting that first good core, so I know your frustration. I really wanted to learn more about going from core, to "blanks", to an actual tool.
If you don't want to keep buying obsidian, learn what the local material is around the area you live. I know that for the area I live in, Northern Colorado, the easiest material to gather is chert.
There are books that help to give instruction on what to look for in the material and how to get the most use. Flintknapping: Making and Understanding Stone Tools by John C. Whittaker is a good book and so is Old Tools-New Eyes: A Primal Primer of Flintknapping by Bob Patten. I have used both of those books as reference myself.
Good Luck