Are these exterior pieces available for purchase or used only?

 

They're still available, but there are some caveats.

The window belt molding should be available, no issues, but the prices have gone up with inflation.

The A-pillar Sill panels are available and extremely expensive. You would need to have the piece re-painted to match your vehicle which adds additional cost. Your best option savings-wise is to repair the original or find a used one from a dismantler/wrecker. Looking at Z1's website, it looks like they now offer a paint match option, before it was only Pike's Peak White. Still, you're looking at about $800 + Tax & S/H for a single side.

Good Luck!
-Icer