Try the Jim Carter catalog, they have a NAPCO section. Also, I think this is the site that actually sells the NAPCO rebuild kits offered periodically on eBay: http://doonan.home.mindspring.com/ddautoparts.html.
I have bought a number of rebuild kits from them, they seem to be very nice. Just make sure you get the right year & sizing (1/2, 3/4 ton, etc.)