I found one behind the military recruiting station in tacoma mall. I found one somewhere else but i'm pissed cuz i cant remember where i found it. I think it was either in the montlake area or somewhere in redmond. Ill let you know when i find it or remember where it was.
There is a large area at Magnason park in Seattle North shore off of Sand Point Wy, it used to be a military base they have the MSF course there sometimes, I would not use it for stunting if there is a MSF course going on.
Montlake parking lot which is right near the IMA. If you're worried about practicing out in public, you can go behind the IMA building where you will find another large parking spot. Granted you'll have to be doing all of this at night (the parking lots are full during the day), there are light posts so you can still kind of see where you are going.