It's funny how these things work out. Here in NW Montana, woodworking supply stores are nonexistent. If it isn't at Lowes, Home Depot, or a handful of local hardware stores (and it isn't), you aren't going to find it around here. A while back, my wife dragged me into Michael's craft store totally against my will. While there, I noticed that they sell baltic birch plywood. The only source in my area. It may be a little more expensive, but at least it's available.
Jim