Honestly, I just make them at home with Hebrew National or Narthan’s dogs, grilled walla walla sweet onions, some cream cheese and dijon mustard. But when out and about, there are a few places I’ve found them.
There’s a hot dog cart (might be Dantes) @ U. Village, and I’ve gotten a mustard/cream cheese/onions combo there before. Also, the awesome Po Dogs (which has a few locations) has a Seattle dog. Our kids love it there, too. My favorite offering, when I’m in the mood for something even further off the beaten hot dog path, is a Wasabi Egg Roll dog. The wasabi sauce has just the right amount of kick to it to give you a little eye water. http://www.podogs.com/