i made it out to Roundrock, Tx this weekend in hopes of getting to navigate some creek beds for materials. due to time restraints, i only managed to visit a landscaping supply company in Georgetown, Tx.
they didn't have a "flint" bin on the premises, but i was able to find some promising pieces of "Colorado River Bull Rock". it was only $0.07/lb and they allowed me to pick out what i wanted from the bins. after about 30 minutes of acoustic testing with a small hammer stone, i walked away with 60lbs of nodules for less than 5 dollars.
only two of the larger nodules gave a really sharp ring when tapped, but this morning i was getting some decent spalls from one of the 'softer' rocks.
i'll post back with some photographs and perhaps someone can help identify what i actually have, because i didn't see "bull rock" on the PA rock resource thread.