DD and EDDL files can be downloaded from the respective bus organization
4-20 mA/HART and WirelessHART DD file download
FOUNDATION fieldbus DD file download
PROFIBUS and PROFINET DD file download
For control systems it is generally better to download the "device package" for the device from the system vendor's website as such packages include additional files over and above DD/EDDL file, such as device faceplate graphics and default value templates etc.
Where can I find and download a DD file?
This page redirects to the appropriate 4-20 mA/HART, FOUNDATION fieldbus (FF), PROFIBUS, PROFINET, and WirelessHART organization. That is, the HART communication foundation, Fieldbus Foundation, and PROFIBUS and PROFINET international (PI)
How do I install a DD file?
DD files are simply copied into the correct folder on the system. Many systems have wizards/utilities to add device types that guides the users, automatically copying the DD file into the correct folder
Learn more more about device integration, device revision management, and DD/EDDL files here