There's a company called J. Stout Auctions that runs out of Washougal, Vancouver, and Spokane that handles large vehicles. Of particular interest to you is that they handle (among other things) Portland's Tri-Met bus fleet, which are generally well maintained and are required to be aged out after a certain mileage.
I've bought many things from them (including my shuttle bus) and they're great folks. If you're after something specific, give them a call - they generally get inventory on a schedule so they can tell you when they'll have new stock.
https://www.jstoutauction.com/
Good auction places for buses?
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