Factional mounts: Apprentice
You can purchase these mounts from Harb Clawhoof <Kodo Mounts>, in Bloodhoof Village in central Mulgore so long as your character has riding skill of 75 (obtainable at level 20). If you are playing as any race other than a tauren, you will also need to have Exalted reputation with Thunder Bluff.
Factional mounts: Journeyman
You can purchase these mounts from Harb Clawhoof <Kodo Mounts> as well, so long as your character has riding skill of 150 (obtainable at level 40). They are reputation restricted like the apprentice mounts.
See also: TCG Card.
Other Kodo mounts
can be purchased with a from Deathguard Netharian.