There are, like, a dozen out there on the market right now.
Some come in sets, like those from Louisa Smith - from her Strips and Curves series of quilts/books:
http://quiltescapes.com/products.html
Scroll down.
These are pricey but come in multiple sizes that fit together, which allows for a lot of variety.
or the Backporch templates:
http://www.backporchdesign.com/templates.htm
Come in different sizes. I see she has a new all-in-one square-up ruler.
You can go to Joann's, where they have 2 piece sets, although a rather large size, iirc.
There is also interfacing by Quiltsmart, where you never have to sew the curve. Drawback is only one size, but depends on your project.
http://www.quiltsmart.com/khxc/index...acing_q_circle
Or course, you can just make your template from mylar, if you're so inclined, and use a square ruler you already have in your collection.
When I taught a curved class, we used variations of all of the above. It's always good to know a variety of methods.
Bottom line, though:
The key to success is really knowing how to sew a curved seam (either pinning, no pinning, rulers, mylar, registration marks, whatever the method) more than anything else. You can have the fanciest rulers - or even have your pieces die-cut on a Go or Sizzex - but if you can't get that curve, it will show it.