You can find some kind of fossil within 20 minutes in Texas. You just have to try different areas. I am giving you information on the Dallas/Ft Worth, North Texas area.
West Fort Worth has abundant ammonites, heart-shaped sea urchins and other invertebrates, as well as NE Texas and the border of Oklahoma/Texas at the Red River. The Arlington, and Grapevine/Colleyville area is pretty sparse for fossils. However, there have been some fossils found of dinosaur, crocodile and ammonites. The Grand Prairie and Irving areas are good for collecting shark and fish teeth, vertebrae, and ammonites.
It is best to try in early spring or fall, and especially after rains, and even when it has not rained, and there is a lot of erosion.
The Eagle Ford Shale Formation produces ammonites with the original “mother of pearl.” These beds, well exposed in cuts along the Dallas-Fort Worth highway at the western edge of the town, (see map) and have a thickness of one foot. They consist of gray to brownish, limestone containing considerable clay, silt and fine sand. In this area are numerous fossils including small ammonites, pelecypods, fish teeth [Ptychodus], and worm tubes or castings. There are also many shell fragments.
Any open bit of soil is a potential site. Try creeks and the slopes of hills were there is no vegetation for potential collecting sites.
If you don’t find here, make sure to go to other cuts in roads and highways and check it out.