Dunno about these being available ready-made... I was looking for something similar a while back as I wanted mannequin hands that were posed in the right position for holding a blaster.
What I ended up doing, was getting some dental alginate off eBay, which is the stuff they use for lifecasts (some are advertised on eBay for making casts of your kids' hands for example).
My plan was to pose my forearm as if I were holding a blaster, fill a bucket with the alginate and stick my hand / forearm down into it.... then use the cured mould to cast a resin likeness of my arm. Unfortunately I never got around to it :rolleyes but I have a couple of links that might be useful for you:
I did exactly that - used alginate and cast both my hands for my USCM armour display.
Since a pulse rifle is quite large and heavy I actually cast my right hand holding the grip (took 2 tries to get the index finger pointing the right direction) - then attached the whole hand/grip to the rifle. The left hand was moulded in an appropriate position for holding a motion tracker. I ended up painting both hands black to match the mannequin head.
BTW urethane resin and alginate don't work well together. Since the alginate is water based you get big blobs of water forming between the alginate and resin as the resin heats up when it cures. Plaster absorbs the water when it's setting. I used hydrocal which is stronger than plaster but if you wanted a resin cast then making a silicone mould of a plaster cast would be the way to go.
I was also thinking that casting hands in flexible urethane with armature wire down the fingers might be useful for semi poseable hands.
alginate is good stuff, we use it at my current (medical) work all the time, and for the movie stuff i used to do, we used it for some frankenstien monster hands we did for a guys project, molded my hands for it (dont know if thats good or bad in regards to my hands )
works really good, fairly simple to use, and youll get exactly what you want if you do it yourself....