In SlicerDMRI (http://dmri.slicer.org), we have a module “Extract DWI Shells”, which is designed specifically for extracting DW images at b-values of interest. This module should be available in Slicer, after you install SlicerDMRI. However, it seems the nightly version of Slicer does not show the extension manager to install SlicerDMRI. I suggest that you wait a little until this is fixed to use this module. For now, you can try what Fedorov suggested on DICOM data.
Regards,
Fan