Roland officially released the R-8 Software Rhythm Composer for their Cloud service. It is a plugin emulation that includes the base sounds and the "Electronic" card. This is the most authentic way to get the multi-velocity layers without hardware.
Roland officially recreated the R-8 as the SRX Orchestra and SRX World cards for their Cloud platform. Not 100% identical but legally sound. roland r8 samples top
If you're looking for the best digital recreations of the R-8 and its rare expansion cards, these are the most highly regarded options: DrumVerse R8 by Reverb Machine : This is arguably the most comprehensive pack available. Roland officially released the R-8 Software Rhythm Composer