You can buy horses in the stables in the bazaar or drogmors (the big lizards you see people riding) in the lighthouse in the Gulf of Gunthak. The cheapest cost about 8-9k, depending on your Charisma. There is no difference between the horses and drogs as far as stats. The white and black horses are slightly more expensive just because people like them more.
The cheapest mount does not increase run speed. Let me say this again. It does not increase run speed. It actually slows you down a little bit due to the mount requiring a little bit of time to get up to it's max speed and slow down to stop.
The mounts go up in speed slightly as the price goes up. The most expensive storebought one is still slower then a bard or pally/SK on their AA mount though, so don't get your hopes up. The speed of the horse (and corresponding price) is determined by the type of bridle for horse and size of drum for drogmore.
EDIT: Damn the chatting and posting at the same time. Makes you slow...
Edited, Oct 18th 2006 at 8:04am PDT by Jiggidyjay