I don't know where you can get a free one. Judy Matheison's books have larger Mariners compass designs but they are not the traditional "split star compass". Her book "Mariners compass setting a new course" has some patterns for 36" compass in it.
http://web.mac.com/jackmathieson/Jud...der_Books.html
Other than that your only choice is to draft one yourself. It is not hard to draft but I have never drafted anything bigger then 16". Judy Matheison's books give complete detail instructions in how to draft as well. I know I sound like an advertisement but sometimes you have to buy a book to get what you want. Her's have been the best I have found. I just tried a google and a several sites make the same recommendation I just did.