Dave, I get 16"x24" panels from Cherry Tree
1/4" Poplar Plywood | Cherry Tree Toys
I physically can't handle full sheets of plywood anymore, so these work out well for me. As with buying any wood product on-line the shipping is a killer, but as your piece count goes up the cost/sheet goes down. I cut 100+ puzzles/year and have never had an issue with core voids with this product. Additionally I've found that it's much easier to cut than BB and easier on blades.
Bob, according to their description all 5 plys are poplar.
I'd be careful with plywood from the big box stores. Their 1/4" plywood typically has a single ply core. This is what burned me on my military plaques.