freecampsites.net
iOverlander
^ these are the main resources I use when looking for somewhere to camp. But they are much more focused on finding spots to bed down for the night between destinations.

In the south, you are probably better off searching through state park, national park, or national forest websites.

Imo, a better strategy is to think of something you want to do while out camping. After all, camping is just sleeping, but less comfortable. What do you want to do for all the other hours in the day? Fish? Hike? Paddle a canoe? Look for spots that accomodate these things, and then go try them out.

Also, the best resource is going to be other locals. Try to find some friends who want to go with you, and your social network will probably provide all the info you need