Well, the problem there is they're a tribe within a tribe. Anything with lots of data on the Salish-Sidhe will give you a little on the Sinsearach, but...yeah. The Salish-Sidhe are already seen as one sliver of the NAN, and now you're looking at a sliver of a sliver, y'know?
NAN (1) will have probably the most in-depth stuff, but also the most out of date. SoNA would be next, and have less info, but only be about a decade behind the curve. That leaves, uh, stuff that's already been mentioned, really. They've got some info in most Seattle sourcebooks (because they're next door, and Seattle sourcebooks tend to touch on nearby neighbors a little), but you're still not talkin' about a whole lot, really.
There's lots of elbow room to do your own thing, is where I'm goin' with this, really.