Hm...it varies, depending on which character I'm working on.
For Durandal, Tycho, and Leela, most of my inspiration came from Destiny concept art (with some Galaxy Fraulein Yuna for Leela); Cortana has a little of of that as well, IIRC. With Haller, I think I was looking at military uniforms, both real and stylized; other things I've turned to include Guild Wars (mostly Nightfall, and primarily for the Jjaro), Deus Ex, and Halo. I've also referenced Pokemon's human designs for when I want something stylized for a non-armoured android.
When it comes to casual clothes, the first thing I look for tends to be vintage mid-90s clothing (since that's when the Marathon games came out); if it's the kinda sci-fi you're going for, you could try looking up costume design from shows like Farscape and Babylon 5. Just putting "sci-fi clothes" or something similar into Google should bring up useful results.
Hope this helps!