I don't know anything at all about resin casting except that you need to make a latex rubber or silicone mold, and then fill it with the two-part casting resin material.

Here is a brief description of the process from Wikipedia:  http://en.wikipedia.org/wiki/Resin_casting

If that looks a little too complicated, perhaps you can advertise in your local paper and get a local hobbyist to make a few for you.  The materials are more expensive than for papier mache, that I can tell you!

Good luck in your search!

Sue

I have friends who use latex to make molds for resin casting, you will need to be very careful as it is highly toxic and you need to know what you are doing with it to be successful.  It would make sense to find someone to help.  Where are you? if you are in the states then get in touch with Puppetsandstuff.com, Sean, who runs the site may be able to put you in touch with a puppet maker in your area who could help.
Alternatively contact a local theatre as they will have details of people who make casts for stage and props.