for resin fittings, your best with a mould from something like silicone rubber , its a 2 part mix that sets as a flexy mould that can take great detail, will flex to remove parts and can be reused many many times
In a silicone mould no release agent is normaly needed for moulding the fittings, but for making the mould a release agent is needed on the master
you can use any casting resin to mould them, the cheapest is a polyester casting resin (quite high smell like `car` resin), the commecrcial mouldings are normaly polyurethane resin (it smells less but is more toxic)
they are all 2 part resins, the polyurethane is normaly an equal mix, the polyester works the same as a fiberglass resin, but sets in a casting slower so it doesnt set too quick
for the silicone and the resins a lot of modelers get stuff from glassplies in crowland street southport , they do mail order
www.glasplies.co.uk/they also do all the glasscloth and resins for making hulls, they have brochures on making moulds etc as well in there catalogues