Denton, Clear and Hickory Creeks are excellent places to look. The Trinity is also full of all sorts of goodies if you can get past the trash and glass.
Dallas is the home of the most fanatical "shark tooth hunters " in the world ! Do some online homework, contact the "Dallas fossil club" ( or similar name like "Dallas paleontological society" ) They do field trips to legal, rich surfacing areas in the DFW and surrounding area. Zero cost, most "clubs" encourage guests to come along.
The point is ( pun intended ) for your goals, to heck with the fossils....just pickup the flint that lies along with the fossils.
In a general word... shark teeth from the Sulphur drainages, flint along the Brazos