Before you click on any shady "DLL download" sites, there is something crucial you need to understand: It is not a piece of software you install like a patch. It is a capability of your graphics hardware (GPU) and your drivers.
If your hardware does support 11.1 but the game won't launch, try these steps:
Stop searching for a mysterious "d3d feature level 11.1 download." That file does not exist.
DirectX "Feature Levels" represent specific sets of GPU capabilities (like UAV slot increases or constant buffer improvements). If your graphics card hardware was manufactured before these features existed, no software download can force the hardware to perform those specific tasks. If