How do you get Longbow Archers?

[caravel]

MTW consists of three eras: Early starting AD1087, High starting AD1205 and Late starting AD1321. You can start your campaign at any of these eras but if you start in the early era the high and late eras will still come to pass anyway. Each era brings about new technology and often sees the phasing out of old. Longbows are one of these units, and are only available from 1205 onwards. The longbows you bribed in wales are the exception. They are the only ones you'll get until 1205.