How do you unlock burning crusade heroics in World of Warcraft

When you queue for a random heroic, you don't need to unlock the Burning Crusade heroic dungeons. However, if you want to enter on your own (or with a group on your own) then you will need to buy the corresponding key from a rep vendor.

Each faction sells one key, obtainable at Honored or higher reputation

Honor Hold/Thrallmar have the key for the Hellfire Dungeons in Hellfire Peninsula
Cenarion Expedition has the key for the Coilfang Reservoir Dungeons in Zangarmarsh
Lower City has the key for the Auchindoun Dungeons in Terokkar Forest
Sha'tar have the key for the Tempest Keep Dungeons in Netherstorm