If you have a "GOOD" wood dealer in your area, ask for Apple ply or Italian flexible ply. I have used both and they are pretty much the same as the stuff I ordered called lite ply. Yet I bought a 4'x8' sheet for about the same cost. Another source is utility panels. They are cheap, not as well made but at you can stack them for thicker pieces. They are a lot lighter than Baltic birch, or other aircraft grade plywood.
http://www.homedepot.com/p/Utility-P...3096/100535208
Buzz.