I'm assuming you meant mold when you said dies. They make molds for 500 grainers that are used in 45-70's. I don't know your bore dimensions, might have to size them to make them work. Would beat buying a custom mold tho.
Look at the 45-70 listings,next to last on page, they're .459's.
http://www.leeprecision.com/cgi/cata.../bullmol2.html