I've bought Strathmore 500 Single-Ply Plate from Jerry's Artarama. They had the best price if you bought at least 25 sheets when I last checked. I gather from another albumenist who inquired that they don't have much 1-ply left and aren't likely to reorder, but if you're not doing albumen, 2-ply or heavier is probably better anyway, and it's pretty easy to find. I've seen it at Pearl Paint here in New York and at other places that sell watercolor and drawing papers.