Nick,
Mate, I'd be surprised if anyone carries Maple in the dimensions you need.
A number of years ago I ventured in to making drum shells (a long story for a guitarist that can't play drums
) and as the majority of "known brand" shells are Maple I sussed out most suppliers of Maple (veneers/ply) in Aust. Most comes in as 0.6mm, however there was also a thicker (but lower grade) which was used for making skateboard decks.
If I was looking to do what you're planning, I would be using the 0.6mm veneer and making my own ply. For a 6mm thick ply I'd cut 10 pieces of 0.6mm BUT make sure at least 1/3 were cut across the grain, then I'd glue them together using TechnoGlue (epoxy) in a vacuum bag setup.
Time consuming but the only way I know of getting what you require.