I know your struggle. When writing my sci-fi comic I kept having that exact trouble and I tried really hard to think of something that was somewhat original. After a while I ended up using Galactic Union and Galactic Allied Forces. Not the most original choices but you know, they fit the story and the world I've been building, so why not.
I think it's okay to choose something that isn't 100 % original because sci-fi is a large genre, so there's bound to be some similarities between different medias. However if you want inspiration on different name ideas then try it from the perspective of the galaxy you're building. Are they a federation or more like an allience? Maybe it's a coalition? Think of the world and use the one that fits it the best. If it's Galactic Federation, then use that and don't worry about it being used before.