Any good automotive electronics shop should be able to rebuild it for you. I have a generator in from a 60 Saratoga 383 now, and had a couple starters rebuilt already for 57-64 Mopars. Cost me in the $65-85 dollar range for starters and generators w/guarantee.
Check in the local yellow pages. If you do not have a core to rebuild, check eBay or your local wrecking yard to get started on a rebuild.