this site has one of the best selections of new castings

go to the bottom to see how to measure

http://www.peterdyson.co.uk/acatalog/Centrefire___Pinfire.html

finding a match to the original could be tough- but you might check with Larry Schuknecth at
http://dutchmanwoodworks.com/

he has a lot of odd parts and fixed a broken pair of hammers for me and you cannot see the repair.

ebay also sees a lot of hammers listed

Tom Carter make hammer screws - you can PM him over on the Parker Gun Collector board