You can make your own electrode cables by soldering a regular wire to a piece of silver wire.
Here are some instructions on how to make that wire from our workshop:
Silver wire is often used in making jewelry, so you might have some luck in a jewelers shop.
As an alternative, you can use copper wire on the plant directly, but it will not work as well, and you should not use it for very long, because the copper can oxidate and rot the plant.