Where to find largish rocks?

I’ve bought myself a chisel set and 3lb hammer and have begun tinkering with stone carving, but I am having some trouble scavenging rocks (I’m not skilled enough or ready to buy real stone for carving). Everywhere I look in Houston to find rocks (empty lots, bayous, etc.) seems to be filled with chunk a of concrete instead.

Where are the best places to find rocks? I find stones the size of a bread loaf to be ideal.