You can find Rattlesnakes at various locations throughout the RDR maps. See the link below to the Rattlesnake guide at the RDR Wiki to show you where to find them.
You usually just find them randomly while riding along there is no way of being able to tell where you will find them
in mexico
You need to go to Game Marketplace. Find Red Dead Redemption. Then there should be a DLC list.
They are in random locations throughout the game world. Once you are in the proximity, a blue circle will appear, notifying you of the one of the horse's locations.
You find them all around the map
at fort mercer
Gaptooth ridge
youtube
snakes birds
ha you noob....play the game and find out
She comes by randomly, there is no certain place you can find her.
South of the border, in Nuevo Paraiso.