In more urban areas, you just go to the abandonded industrial zones. It's not without risk, but neither are those barns: my brother-in-law almost got his silly self shot for poking around what he thought was an abandoned farm because he was interested in buying it.
Otherwise, get in your car and start driving. Look for smaller and smaller roads. Unless you're on Manhattan Island or some similarly superurban area, in less than an hour you can get into country which used to be farmland but no longer is in almost every part of the US.