Surface Reflectance processing for Landsat 4-8 Collection 1 data can now be ordered through EarthExplorer. EarthExplorer has been updated with Collection 1 Higher-Level data sets which contain Landsat scenes that are available for On-Demand Surface Reflectance Processing.
Sources/Usage: Public Domain.
See here for more information about Landsat Surface Reflectance products.
*Note: Not all Landsat 8 scenes have been processed to Collection 1 yet. As scenes are processed to Collection 1, they will become available for Surface Reflectance processing. Landsat 8 Collection 1 processing is expected to be completed in June 2017.