You can buy it for 8500 dublons in Port Royal from East India Company Auditor
The main purchase condition is:
1) Reputation with East India Company at least: Celebrated
2)You must have 8500 dublons
"Inscreibed on the handguard of this excellently balanced blade are a set of helpful directions for surviving a swordfight, courtesy of the East India Company."
You must be lvl 50!