Dear Paul,
I am not aware of a source of 15-minute load data.
The NREL OpenEI website hosts a
database of typical hourly load data for residential and commercial buildings
. This data was generated by a computer model, and is matched to the NSRDB TMY3 weather files, which makes them well suited for use with SAM. SAM 2013.9.20 (scheduled for release tomorrow) includes a sample file with a SamUL script that you can use to download the load data directly into SAM.
Best regards,
Paul.