If you live in the US go to the PSI website and send them an e-mail. They usually respond within 48 hours and they can let you know of a local retail store or distributor that carries it.
I had to do that to find a source for the kneadtite "green stuff" because I got tired of paying an exhorbitant amount to Games Workshop.
I should have added that information. Click the link at the top for their online distributors. You can also go to a hardware store and see if you can find it there under the plumbing section.