I finally got an email response from PSI...
They carry the parts for the bolt action pens, but they do not 'sell' them individually. However, they graciously shipped me a replacement bolt so that I can repair my customers broken one. When I asked about buying them (so that I could keep some on hand) they responded with a statement to the effect that 'replace them for their customers on an as needed basis, but do not stock the amount that would be required for all of those that might order them just in case'
so send them an email, or give them a call, the customer service was top-notch (i.e. I got free parts and didn't even pay for the shipping)