Many of the suppliers will list the dimensions of their sporrans, but there are very few patterns I've found. I've made a couple just from looking at pictures and drawing up a pattern on poster board. It works best if you have one in hand to copy, so you can see the details.

Some of the folks in the DIY section post good pictures of their projects "in the works", so you can get an idea of construction techniques. It is a pretty simple product, but like anything, you can take it an infinite number of different ways.

Good luck on your venture