There is a wide variety of different websites. However, i got my clay from a place called Hobby Lobby.
Ive done this type of mold making process. MAKE SURE you fiberglass the inside though!
You also dont need an epoxy. You can add a layer of blue painters tape (oil based clay will stick better to it)
Remember, for more detail, use the harder clays. After adding the desired amount of detail then you can use brushable rtv silicone on it. It WILL work fer yah.

Soon you guys will see my CQB resin helm