gear wheels

Can anyone help. I want some small metal gear wheels for a 2:1 speed ratio, say 12 and 24 teeth or 15 and 30. I would like them to fit say a

3 mm or 4 mm shaft. I am making a steam engine from K Harris's book on Model Stationary and Marine Steam Engines and need to make a gear box with a 4:1 ratio ( 2 x 2:1) I don't have the expertise, or equipment to make the gears. Hobbies used to stock brass gears, but I don't see them in their latest catalogue. Does anyone know of a supplier? Eddie P