I remember a discussion on this once before, no one seemed to have a source for this. All of the available baltic was 5 ply. I did a job for a design student and she brought in the wood, it was called "Craft Plywood" by Midwest. Apparently they make wood thins largely for the RC airplane hobbyists, so while craft stores have their balsa and basswood, this stuff in 12"x24" and some others are sold in Hobby shops. There's no indication that it's Baltic Birch, but it does look like it, and the 1/8" she brought in cuts like butter with no charring.
http://www.midwestproducts.com/