For anyone else who might be in the same predicament, the fine folks at Homebuilt Airplanes came up with a couple more sources. B&D International carries 5′ x 5′ sheets of Finnish birch aircraft plywood, and will cut and ship UPS. Boulter doesn’t carry as wide a variety, but might be good if you’re closer to the east coast than the west.
The price at B&D is higher than Aircraft Spruce. If you price it by the square foot, it’s not terribly higher. 13%, on the 1/8″ sheet I need. Not terrible.