Several kits on the market, but one can make up their own mixtures, with
great success and less expense.  Technovits has one, also Polysciences
has a GMA kit.  If there is a high volume of work, bulk do it yourself can
be cost effective, plus you can adjust hardness etc of plastic.
Patsy Ruegg has the method to die for, it is also published in Laboratory
Medicine many years ago.  Patsy, arise again (nothing like volunteering someone
elses expertise!)

Technovits (????#) was a dream to section, but never tried working with it.
in terms of embedding, etc. Hopefully Linda Jenkins will comment on this 
plastic, she gave an outstanding workshop on tissues embedded in this.

Polysciences also works fine, the JB-4 kits.  

Gayle Callis