Elementium depths, SW corner of TWH...though you could just do your TB dailies with your new pet, too.
I level mine in the twilight highlands where the fire elementals are. They spawn quickly and I can just run around in a circle killing the with little effort. Not to mention volatile fire is worth 20g each on my server so i make a bit o change as well.
Yeah, prior to Cata, probably the most common way hunters leveled pets was just doing randoms. Obviously this isn't as efficient these days. The other popular way was grinding in certain areas you had to do things in anyways. These places included things like grinding mobs while farming (leather, cooking mats, etc) or dailies (Argent, Hodir, Ebon Blade, etc) for rep. So just do that in whatever zone you need to be grinding anyways. Just be thankful it's now only 3 levels we have to grind.