I never saw one
for sale in
Cartagena, but navigable electronic versions of his
charts are widespread there, along with free
software (Seaclear) to view them. Those could get you to the Western San Blas, where you can buy a copy of his guide at Isla Elephante (sp?) at the "bar" in the West Lemmon island group.
Additionally, you may luck out and find someone in Cartagena who is leaving
Panama and wants to sell their guide. Also, it is possible that the guide exists in Cartagena - there are a couple of large bookstores in the
shopping mall there.
Mark