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/RIMLPBAAR_N.B, is available at:
http://www.federalreserve.gov/datadownload/Output.aspx?rel=H15&series=b2763fc01e872c23bc1aa1ef99a9a8de&lastObs=&from=&to=&filetype=csv&label=include&layout=seriescolumn