Substance Source is a vast PBR library available to Substance Subscription members. You can download materials from Source directly in Unreal Engine. 

Source in Plugin versions 4.25.0.5 and 4.26.0.2:

Source action will now direct to the Substance Launcher - Source tab and allow you to send assets direct to UE4.

1. Click the Source button and the Substance Launcher will open (or be prompted to download, if the Launcher is not already installed)
   1. Substance Launcher will need to be downloaded from the Licenses tab when logged in to your Substance account at https://store.substance3d.com/user
2. Browse the available assets and click the Unreal Engine Logo to send the asset direct to UE
3. The material will be generated with Substance texture inputs and ready do apply to a mesh. 
   

Source in Plugin versions lower than 4.26.0.1, 4.25.0.4 and lower: 

1. Click the Source button at the top of the UI to launch the Source Window. 
2. Login to your account
3. Browse the materials and click the download button to add the material to the Content Browser
4. The material will be generated with Substance texture inputs and ready do apply to a mesh