Quadro & GeForce Driver 346.03.15f15 for OS X 10.11.6 (15G21013).
(Here's a direct download link.)
The driver page text mentions a very old CUDA v7.5.30, but link goes to latest CUDA (which does not list 10.11.x support). If you already have CUDA installed, check the prefs for an update. (The last CUDA version that listed OS X 10.11.x support on the download page was CUDA 8.0.90, but the D/L page listing isn't always accurate.)
Because of improvements in macOS security, the Security & Privacy Preferences may open during the installation process. If it does, click "Allow" in order for the NVIDIA Graphics Driver to load, then return to the Installer and click "Restart".
The "Additional Information" tab on the download page has a step-by-step install guide.
(If you can't select the page's 'Accept' terms/conditions button, here's a direct download link.)
On July 9th Nvidia posted a new CUDA 396.148 that lists support for the above 10.13.5 driver.