FOSECO
formatting link
They make custom lost foam patterns.Tooling isn't cheap. Price based on volume. Expect to pay $20K plus for tooling depending on the complexity of your pattern.
You could try pouring urethane into hand made plastic moulds as one-offs for hobby use.
Or buy blocks of Styrofoam and cut with a hot wire. Do only in well ventilated areas as this produces nasty chemical by-products.
Or machine your part on a CNC...
Can anyone point me to companies out there that can produce parts made
> out of the same material that common sytrofoam dinner plates are made
> from? You know that white, relativity rigid, smooth skinned,
> lightweight and thin foam-like material. I would like to get an idea
> of how much the setup charges would be to mold small parts (< 20 sq.
> in.) out of this material. After searching a bit on the net it looks
> like the process is rather complex and out of the reach of the
> hobbiest. Any companies out there that mold this stuff in medium
> quantities? Thanks in advance for any help... please post answers to
> the group if possible.