The X and Y value should be interchanged, ideally you want to use the vegetation indices to predict biomass outside the study area and therefore using vegetation index as the "X" value in the regression equation and get the output interms of biomass
Most of this data is public depending on what country you are in. If you are in the USA a simple google looking for Raster data may be useful. Dozens of universities and government agency's have this kind of data open to the public.
