Try calling velocity in Alexandria and Phoenix Bikes in Arlington if you have no luck here–sometimes they have random stuff in stock. And post on the Facebook group DC used bicycle marketplace. People there have some random parts and are usually willing to part with them for cheap.
To add on to what Emm suggested: If you’re looking at Phoenix and don’t mind getting a little weird you can grab a bmx seatpost with a clamp and then shim it up to 24mm.