I have not seen any that large recently....maybe going to a LQS and going through their patterns/books/magazines? Or the library?
I have a Mariner's Compass book that I checked out from the library that gives step by step instructions. The patterns in the book are for up to a 32inch center medallion and are paper pieced. It is called Mariner's Compass Quilts, Setting a New Course written by Judy Mathieson