Value Village is always where I head when I'm looking for anything costume related. Or, more precisely, when I'm looking for anything period. It's kind of a long shot when you have something really specific in mind, but you never know what you're going to find.
I prefer the one in Ballard/Crown hill, however there are also large locations in Lake City, Capitol Hill, Redmond, Lynwood, etc. If you can't find what you're looking for specifically, don't fret. They're stock full of enough absurd grandma sweaters and purses to make the trip productive regardless.
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