Hmmm. I don't know of any sites off the top of my head, but I bet some college drafting classes might, and a lot of college classes put their materials online. I remember making odd shapes like that a lot in drafting class, although the class I took was pencil-and-paper.
For what it's worth, here's a quick video of how I'd tackle it:
http://independentdeveloper.com/files/makecurve.wmv