Q: How to replace supercharger idler on 1998 Buick Park Avenue

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The supercharger belt is missing and the lower idler is missing.
(1) Answer
You can buy these parts at the dealer or at an auto parts store. You don't say whether these parts were on the car when you got it. I'm suspicious that the supercharger is faulty and someone removed the parts.
If you want to find a specialist, here's a directory link for you: