I worked briefly in a satellite lab, and we did indeed have duplicate parts for most things.

Any space telescope with a mirror, is a different story, e.g.

https://science.nasa.gov/mission/webb/webbs-mirrors/

The RST has an 8ft mirror. The mirrors have to be ground and polished in a clean room to tolerances that are the highest in the world. I vaguely recall it's a square law or worse - for radius-to-time-to-polish. So you're paying for time, and you're paying for the expertise of one or two custom built places that can manufacture them.