Primal Shadows sell for about 30G on my server. Two mobs with higher drop rates for the motes to make the primals are found in HFP. They are:
Collapsing Voidwalker
Creature: Collapsing Voidwalker - Thottbot: World of Warcraft
and
Vacillating Voidcaller
Creature: Vacillating Voidcaller - Thottbot: World of Warcraft
these mobs are level 62 and are easy to kill but there are 2 issues you'll need to get over.
The Collapsing Voidwalkers Are only level 62 but have a drop rate comparable to a lvl 69-70. Though they are easy to kill, they start to collapse at about 10% health left. It's basically a suicide spell. When they finish the cast, they explode doing a lot of shadow damage. A lot more than a lvl 62 should be able to do. It will hurt if you are around this level. They will also explode if you kill them too. So there is no way around this. Best thing to do is not be near them when they die.
Hunters can do this easily. I got a 2nd pet for grinding these, I got a cat because of the high health and can still hold agro. Max health and Shadow resist. I'm less concerned with armor.
Grinding on the collapsing VWs, I get them to about 15% or so health, then hit it with Cuncuss and recall my pet. This gives kitty a chance to get out of the blast radius, at this point, you will most likely trigger the collapse spell, the VW stops moving to cast so with a little practice, you can get this timing down and not take any damage. You'll be farming Shadows as fast as anyone in SMV. faster if you are 70.
As far as the Vacillating Voidcallers go, they have the same ability as the VWs at the Broken Mana Forge in Netherstorm. They will learn your magics, become resistant to them and then use them against you. This is no big deal for hunters. Just deal physical damage only. Don't use any stings or arcane shots. Use steady, auto and concuss only. They will then only have shadow bolts to cast. The shadow bolts do a lot of damage for a lvl 62 so just be careful, but it's nothing you can't mend through.
Their drop rates are posted as 40 and 30 percent respectively but I find that hard to accept. It's probably a bit less but ti's still a pretty good drop rate. At least in my opinion, you'll get a better drop rate than any other non-elite mob that drops motes. As a leather working Hunter, Primal Airs are very necessary for me. I don't farm Airs, I farm Shadows and sell them to buy airs. This way is just faster. These 2 mobs also seem to drop a lot of greens for me.
Good Luck.