There is what is called Triangle in a Square template, and I think Eleanor Burns has a template for it. (Or someone has as I've seen it in catalogs.) Basically you can do it yourself. Make your square the size you need. Find the middle of one side, then draw lines from it to the opposite corners. Reverse the pooint of the triangle in the next square. Sew the two squares together with the base of the triangles matching and you will have a diamond.
I hope this helps you. If you need clarification, let me know.