Hey There! I believe you can get some artist to help you with 3D, i have a friend of mine who helps me with the 3D and level design while i code the game, each one of us thinks differently but i believe you should avoid to use any level prefab out there and put your level idea on paper, then get somebody to help you with the design of it, cause you are right, making a good level is very hard.
If you are solo developer and you are starting my recommendation is that one, other solution is to work hard in the level by your own hand, something i do mostly myself and trust me is not that hard, but something incredibly frustating, however if you focus in make a fun level and do tons of testing, you will make a great level at the end.
By the way, if you want to use level prefabs, i know Asset Store from unity, i never heard of FPS level garage.
Don't give up! Good luck