How do I know what fits? I have seen all kinds of different stuff in sigs while looking around on here, different brands of intakes, exhaust components, underbody bracing, pullies, but no site to go and look around. And I really want to find some springs, this thing rides waay too high!
Your first place is here really. When people do mods, they generally post where they did buy stuff plus there are stickies with part numbers.
This is a good starting point for springs:
Infiniti FX Lowering Springs - InfinitiScene.com
H&R and Eibach are the two available ones. I think majority uses H&Rs because they're a little more comfortable I believe. I got mine and a friend's H&Rs from ebay. Just make sure you get the correct part.
Here's a good info thread on several facts. You can also find stuff on exhaust:
FX: Did You Know? (FX Facts) - InfinitiScene.com
Apart from the threads, there isn't much main vendor directly associated with the FX. Stillen (stillen.com) is one that has several parts but IMO it's a little expensive. If you want exhaust and don't mind getting some extensions done on it, the best places to find a used one are my350z.com and g35driver.com..
for bracing, this is the starting thread:
GT Spec Suspension Bracing - InfinitiScene.com
There were two group buys on the braces. They are willing to do one more on the whole stuff. Search for gtspec or "gt spec" and you'll see them right away.
As for the intake, Stillen, JWT Pop Charger and LMS are the three most common. There are lots of threads on these as well..