The H.15 historical data is available through the Data Download Program at:
http://www.federalreserve.gov/datadownload/Build.aspx?rel=H15

This series, H15/H15/RIMLPAAAR_N.B, is available at:
http://www.federalreserve.gov/datadownload/Output.aspx?rel=H15&series=64d2cda8bbf1712f9ffe7e31238b9618&lastObs=&from=&to=&filetype=csv&label=include&layout=seriescolumn