Google search, punch in any location followed by "campground" and you'll get just about anything you want to know.
State Parks? DNR web sites for each state.
KOA web site, Good Sam web site, and a bunch sites specific for private campground exist for just about any location.
Then there is the BLM web site for Federal campgrounds and the Army Corp of Engineeers. Not to mention private organizations like Elks, American Legion, and a host of Church organizations that have directories for their unique organization.
When all else fails, there's always good old Google. Just type type in "WalMart" and your selected city!
Since I'm from Indiana, here's one for private campgrounds:
Click here.