What's the best way to prepare plastic bumpers for re-painting? Is there a paint stripper available for the job? Or is sanding the best way to go?

Are you just preping the Bar for a respray? If so, just sand it down, don't remove the paint layers as thing will help bond the new paint. If you go back to plastic you will need to plastic prim before painting.

BIG NO to paint stripper on plastic

its even corrosive on some aluminium wheels