The place Neil was mentioning is called Duthie. It is awesome for jumps, but in my opinion the the XC isn’t great – they seem to have focused more on trying to cram as much mileage in a small space rather than flowy trails.
Unfortunately I don’t ride much XC stuff, but if you are entering some races then I’m sure you’ll meet people. This is kind of how it works here and then you start to find the hiddden stuff.
Other places to look for are Tiger mountain if it is open (chance of seeing bears and cougars) and Tokul.
Good bike shops in Seattle: Big Tree or GHY. Greggs is ok, more like Edinburgh Bike Coop tho, actually they have a XC race team I think, so perhaps worth chatting to them.