This is how I made my 5000k gold for my Epic Flying Mount on all 4 of my level 70s. Sharing it now because I don't know exactly how many people happen to read these forums
First things first, I haven't tried this as anything less than level 70, so if it doesn't work for lower levels, don't blame me, I never said it would.
It's actually quite simple, what you are farming is http://www.wowhead.com/?item=1973 .
The best place for it? Why, the Maul! In case you don't know what this is, its the PvP Arena in Dire Maul. All 3 of the Rare Spawn bosses have a chance of dropping it. The reason this sells for so much is simply novelty. That and all the new people to the game that rushed to Outlands have never seen it, because it doesn't drop in Outlands.
Sorry I can't say more about it, because I don't really know what else there is to say, its pretty straight forward really. I did a search and couldn't find this simple information anywhere, so I thought i would just go ahead and post it.
Also, I don't really know the spawn timers on these guys, it appears to be random. I usually just farm Dire Maul while I'm out there for Large Brilliant Shards anyway, since they sell well, too!
Hope this helps at least one person!