Finding these side covers can be difficult at times. It will also depend upon the year of your bike. I believe that there are three different side covers due to the different electrical systems/stators. These are:
Model year 1987
Model years 1988-2000
Model years 2001- Present
Each one of these designs will bolt right up to any bike in the range listed for that design. Side covers from a different range can be made to work on any year but will require additional mods including stator/flywheel/wiring harness/connectors/etc. - possible, but not very practical for most of us.
Hope this helps.