I'm level 45, night elf warrior. I was wondering when i can buy one? I've trained twice at the Stormwind flying trainer but when i go back to Darnassus i cant seem to buy one?

You can learn your beginning riding skill at 20. and epic ground at 40...but you can't ride horses until your exhalted with SW so you have to go to darnassus and get your nightsaber. Flying is not until 60 and epic flying at 70.

Well you can't fly until level 60, so if you are trying to buy a flying mount, no, you won't be able to get one.

You should have had a ground mount at 20. Talk to a riding trainer IN Darnassus, the mount vendor will be right next to her so you can buy the appropriate mount.