It was one of the rewards from a game a while back. So check your gift box first. Just type in Big Maple Leaf Tree in the search bar to see if it is there.
If you don't have it in your gift box, then go to the Store. You should be able to see it under New, if not look for the search bar for the store. I typed in Big and the first thing I saw was the Big Maple Leaf Tree. It Cost 12 RC to purchase.
Getting the product from trees is not easy unless there are 5 of the same tree on the farm you are visiting. Crops are usually easy because, as a rule, people plant more than 5 plots of one type for harvesting.