Here is a map if you didn't buy the game. if you did then the areas marked with "danger" are the hideouts.
For these just ride up and then they will initiate themselves.
Twin Rocks: America
Mission requirement: New Friends, Old Problems
Objective: Kill all the bandits and reinforcements
Reward: Volcanic Pistol
Pike's Basin: America
Mission requirement: Justice in Pike's Basin
Objective: Help rancher return his cattle by killing the bandits
Reward: Sawed-off Shotgun
Gaptooth Breach: America
Mission requirement: Man is Born Unto Trouble
Objective: Find treasure chest
Reward: Springfield Rife
The link posted above came from xbox360achievements.org
Note if you finish all the hideouts in the US within 24 hours game time you will unlock the Marshal's outfit.