In small quantities, any home supply place such as Menards, Home Depot, etc. will have Maple. You can also go to any of the hardwood supply houses that supply raw wood supplies to homebuilders. Any good sized city will have one, even some regular lumber yards have hardwoods or can get them. There are also many online sources under hardwoods.

Years ago, when TopFlite maple props were popular, Sid Axelrod sourced some maple from old flooring from torn down buildings. It was well seasoned and stable.