31renrub

The poster is outside one of the doors of the Saint Denis post office. It’s titled “Rare Orchids Order” and is found in the “Notes” section of your satchel once you’ve picked it up. According to the RDR wiki, you can only pick it up after completing Algernon’s quest, but BEFORE loading a new save (so, you have to get it before saving the game and then loading said save). No idea if that’s true or not.