Yeah I'm afraid to say that it's going to be tricky in Portland; however, here are a few places to start:
A Better Cycle: they're in Portland on 25th and Division, google should get you an exact address and they specialize in used bikes...will they have a cruiser? I couldn't say.
The Recyclery: Portland, SE 10 and Madison. Another used parts specialty store, these guys might be a good place to start since to my thinking they have a higher chance of having cruisers.
Really Good Stuff: 1322 SE Hawthorne Blvd. These guys are not a bike store, but they do sell some random bikes out front. It changes (I once saw this sweet little stingray there) a lot but I know I've seen cruisers there before too. One word of warning (and I repeat) they are not a bike shop. If you buy a bike from them have it checked out by somebody who knows what they're doing (could be you for all I know), or take it to a shop and let them check it out. Actually the Recyclery would be a good place for that, they're located very close to each other.
^^Good suggestions, I'd second the Recyclery and A Better Cycle, both have connections to ZooBomb. I've seen old cruisers at the Recyclery, but they weren't necessarily cheap. Really Good Stuff isn't a bad suggestion either, I've seen cruisers there too, also not cheap; and this is not a bike shop so everything is going to be 'as-is', they are just a general resale shop that highgrades auction / estate sale items, and sometimes they ruin original items by 'fixing' them.
Maybe CityBikes on SE Ankeny at 7th? Or the Community Cycling Center on NE Alberta at 17th?
A longer shot is Sellwood Cycle Repair www.sellwoodcycle.com. They sell lots of used bikes, but I'm not sure if old cruisers are really their thing. Their website has an up-to-date list of their inventory.