the thread for screwing in the pedal has been stripped on right side. Any way to avoid buying a whole new crank? Put a new pedal in, and followed advice not to screw it in too hard. Unfortunately, I made it too loose... and the thread on the right crank is now almost completely stripped because the pedal was moving around and abrading it.

Is there any way to replace just the right arm?
Cranksets are typically sold with both arms, not just one of them, so the exercise of finding just a right arm that is compatible is mostly academic (the only case I can think of that's somewhat reasonable is a parts bin at a bike co-op or shop which has one, but this is still unlikely). You're more likely to find the whole crankset instead, in which case there isn't really a reason to not install both of them.

That being said, on a square taper bottom bracket, you can install different square taper crank arms on the left and right (different sizes, different makes, etc). You can mix hollowtech 2 crankarms on the left and right as well. So on, so forth -- the compatibility is determined by what type of crankarm you're running.

If you have an autoshop nearby or a bike shop, you can also try to helicoil the crankset. This puts a new set of threads on for the pedal.

Also, note that if you don't have a crank puller, you're going to have to borrow one or likely go into a shop unless the crank is self extracting.