Start farming for your mount just one level before 40
You can actually start at 37, 38 if you have high dps!
Easily generated the 90g needed for your lvl 40 mount in 1/2 a day!
Just north of the Gurubashi Arena and just west of the Jungle Stalkers, there is a small area where the "Cold Eye Basiliks" dwell.
The rate on greenies drops on this guys is extremely high, also, even the grey items that they drop is unusually valuable vendor trash.
But... almost 5% of the time they drop a green trinket called "Cold Basilisk Eye" This trinket, is virtually silly and worthless (equipped and use slows a target's moveent by 15% during combat).
However, it is one of but a few trinkets that can be equipped at a character level of 40. Because of this relatively low required equip level, noobs will go ga-ga and this little trinket will fetch between 5 and 10 gold very quickly.
Put all of them on the AH for 5g bid, 10g buyout and the majority of them was sold at buyout.
In a 2 hour period, those basilisks dropped 23 of these trinkets.
After 3 hours in ah, you have arround 155 gold!!
Not only you are able to buy your mount, You're able to pay for all new lvl 40 spells (which at 40 is quite an expensive class for spells).
No fluke, silly trinket, silly people buying it, but, you have your mount at 40 and cash left over!!
Finally, another added bonus, if you are a skinner, you also just made a bunch of heavy and/or thick leather. Not bad for 1/2 days farming.