Since this is a legacy version (1.8.9), the mod is often bundled in "Ghost Clients" or specific utility packs. You can find standalone versions on: Sk1er.vi (Trusted community developer for 1.8.9)
While there isn't one definitive "Auto Place" mod for Minecraft 1.8.9, there are several high-quality mods that provide automated or enhanced block placement features for that version. These are particularly popular for and creative building. Top Mods with Auto-Placement Features for 1.8.9 auto place mod 1.8.9