Coat front buttons should be 3/4 of an inch in diameter while all other buttons should 5/8 of an inch in diameter. As for button sources, all of the sources mentioned above will do.
I strongly recommend Corozo buttons only of you want to match the color of the buttons to the primary color of the garment. Otherwise, stick with the materials mentioned that the other members already recommended.
I strongly recommend Corozo buttons only of you want to match the color of the buttons to the primary color of the garment. Otherwise, stick with the materials mentioned that the other members already recommended.