Tae -
It might be worthwhile to check some of the Scrapbooking shops, perhaps Michael's - they have lots of different shaped paper punches with decent steel cutters in them. My wife has several different ones, snowflakes, geometric shapes, etc. They come with the steel cutter inside a hard plastic body - the paper slides in the bottom and the cutter is depressed with a short plunger from the top. You could try a small, inexpensive on and butcher it apart to get at the cutter to try on leather.