On the galaxy map, you can enter the nebula's in-game name in the search field (it's easier on Odyssey) and focus on its center. Their true centers are the "virtual" systems which made the nebula, but we can't jump there nor target them: all we can do is to center the galaxy map there. However, it is easier to eyeball whichever system might be closest from that camera position.
The
Catalogue of Galactic Nebulae also includes the closest reference systems for all known nebulae in one place, and you can copy-paste the nebula's in-game name from there too. Some of the reference systems are more precise than what's on the GMP (or what was there at the time I made the list), but for most practical applications, the differences shouldn't matter much. Although I'm curious, what do you use the distance(s) for?