Actually you probably won't find a mail order company who sells bonsai pelargoniums.

Pelargoniums are soft, very soft, wood and might not do well as a bonsai.

You could try by yourself, using a pelargonium that looks small. Bonsai means keeping roots and top growth cut so the plant grows smaller and would produce smaller flowers.

Cutting Pelargonium leaves in half would likely not make them smaller as with other plants. Even cutting roots might affect the Pelargonium in a bad way.

However, try it and see if it can be done.

Good luck!

Nancy