<p>Is there any commonly printed plastic which I can buy that might be transparent to UV light? </p>

<p>I wish to print a mould, then pour in my plastic which requires a UV light to activate the curing process. </p>

This is more of a Chemistry question, but seeing as we love 3D printing with exotics, here are a few.
Topas
olefin copolymer
From the mfr description page,
TOPAS cyclic olefin copolymer, or COC, is an incredibly pure polymer -
in fact, it's purer than most grades of medical glass. Unlike glass,
it has a non-ionic, inert surface to minimize reactivity,
denaturation, agglomeration, delamination and other traditional glass
concerns. And when it comes to maintaining purity, TOPAS medical grade
plastics can be sterilized via all common methods. Leachables and
extractables are extremely low. Reduce risk and increase performance
by maintaining the benign, protective environment that TOPAS COC-based
devices provide.
Medical grades of TOPAS COC are extremely clear, and are optically
suitable for replacing glass in many applications.
I'm not sure of its melting point, or of the speed of solidification (which affects extruder rate, motion etc).
Recommended at
this Chem.SE question
, PMMA and others
edit
Because answers there have links of their own, I"m not repeating the various technical leads available there.
Now you will have to investigate their melting points and flow rates, etc. to see if these can be coerced to function in an extrusion printer.