Usually there is no problem finding "Fantasy" themed , because of the popularity of D&D games. But in the last 5 years or so I have noticed an increase of Sci-Fi themed items. Mainly i think due to the re-newed popularity of Future-moderan superhero genre.

My two main go to ways to add ambiance is:

https://syrinscape.com/ (Syrinscapes)

And

http://www.darkravengames.com/audio-products/4578987775 (Darkraven games soundscapes)

Darkraven games is starting another kickstarter for their "Orion" series of soundtracks. Which includes a hover car ,city scapes, space scapes, mech battle etc

and they even are providing some promo tracks

Still the Star Wars soundboards are always good in a pinch

I've had some luck with http://www.ambient-mixer.com/

For example: http://science-fiction.ambient-mixer.com/inside-the-starport

Try also Tabletop Audio . I use a combination of that and ambient-mixer.com along with Star Wars music.

I just created a mix of "Ambiance" and "Battle" music from all six films for my games. They're in a pair of folders, and I just set WMP to "Repeat" and hit "Play All". Then when I switch to a battle, I'll switch music.

I just created a mix of "Ambiance" and "Battle" music from all six films for my games. They're in a pair of folders, and I just set WMP to "Repeat" and hit "Play All". Then when I switch to a battle, I'll switch music.

I do something similar. I have two different iTunes playlists, one called "Steady" which is calm or suspenseful music and then one called "Action" which is pretty self explanatory.

However I actually don't use movie music as I find it's very specific to certain scenes and quite jarring when tracks change. Instead I use a blend of Star Wars video game music as I find that's a bit more suited for unspecific actions, since obviously that's what it was composed to be. But my group are all fairly young and grew up on Star Wars video games (we're in our twenties) so the music is still fairly iconic and "Star Wars-ish" for us

I've been using the soundtracks to X: Rebirth recently, mostly for space travel and suspense.