You mention petco and petsmart, but have you checked out any of the ma&pop shops around your area? I haven't been to Spokane in years but Washington as a whole usually has a pretty good selection in each area.

A quick google search suggested:
-"Aquarium Solutions"
9516 East Montgomery Avenue #18, Spokane Valley
-"Northwest Seed and Pet"
2422 East Sprague Avenue, Spokane

I'm surprised more aren't listed, but I would imagine that there are a couple, if not in Spokane than at least within a small drive time.