Rhyolite can be tough and not the easiest stone to work if you don't have much experience. But, if you have a source near by then use it! It can have various grades in it and may have weathering, freeze cracks, seams, etc... the best bet for you would be to test some pieces bring a big hammer and prepare to bust some rock! You may have to break some old layers to expose some new and fresh layers. Some other ideas for knappable rock is check for quartzite, glass, toilet tank, ebay, Neolithics.com, facebook, and type in web browser buy flint knapping supplies, I know in the UK that there are several sources and the flint that is in the south can be shipped to u with no worries. Oh if you are at a stone source keep eyes open for artifacts of any and all stone tools, flakes, and debitage...its possible that stone age tribes may have used the source thousands of years ago. That would be cool to discover! If you need any more help or ideas let me know? I have plenty more... oh before I forget... You can try looking into rock formations, geology clubs, gun flint makers for black powder gun, and plain old networking.. you my find that there are other flint knappers near by...
Cheers,
Russ