Until the Gridded Lightning Strike Density Product becomes operational 
on May 21, 2015 1500 UTC, the grib2 data can be found in the following
location (both links go to the same place): 

http://ftp.opc.ncep.noaa.gov/grids/experimental/lightning_density

or

ftp://ftp.opc.ncep.noaa.gov/grids/experimental/lightning_density