Here's a link to look over. It will depend on the size you want.
Here are some free shamrock templates you can cut out of paper, cardboard or even the plastic page protectors.
Shamrocks are basically three hearts and a stem.
Find a friend with a cricut machine where they can cut one for you! You can also look on Etsy for a stencil, sometimes a custom stencil, so you can use exactly what you're looking for,