Recently learned slap bass, one of my major roadblocks is my right hand gets tired, and because of that the slaps get sloppy.

I know practice will help, but for times when I don't have a bass around me, are there any slap-hand exercises I can do to help strengthen my hand?
Slap your thumb on hard pieces of wood, metal, etc. for long periods of time to callous that baby up. It will be easier to hit single strings after that. And simple hand exercises with weights should do the trick for strengthening your hands.