Both of them are in Auburn. Italian Motor’s shop in Sumas is massive (for a kart shop), but it’s a bit of a hike from Gig Harbor. www.imusa.com
This should get you started. Let me know if you need more info. Incidentally, most of the karts on the local Craigslist are garbage. You’ll be WAY better off talking to one of the above shops, especially with your needs and planned mods. Steve and Italian Motors always have some good used chassis hanging around, as their race teams tend to race new stuff and then sell them after a few races.