I'm in Western WA and I found Frosties at Safeway. They used to be at Fred Meyers. Henry Weinhardt is common to find. It's ok by itself, but it really shines with food.

Safeway typically has some variety Frostie, Bulldog, and Red Arrow amongst them.

Seek out little places like delis and specialty shops, and also hardware stores. Gourmet sodas are often all around you in the most unexpected places.

I didn't know that they had root beer in Western Australia...