Air and fire are both commonly used, and hard to get. The drop rate of fire off of fel iron nodes is not very high, and air motes don't come from anywhere but elementals. They go for at least 20g/primal on most servers, sometimes as high as 25 or 30g.
Water is also commonly used, but it can be obtained through fishing and through transmuting cheap earth primals so the supply is higher and the price is cheaper. It's still pretty good money.
Earth comes from every type of outland mining node, so it is the least expensive at 5g or less a primal. It's not worth the trouble to farm it to most people, but you will at least not have any competition if you decide to farm elementals. I got at least 50 of these primals in the course of skilling up my mining to 375 and farming ore at 70. I got around 8 or fewer fire primals by comparison.
Shadow is pretty cheap, as it comes from demons, and a lot of people farm them for Aldor rep items.
Mana primals come from mana thistle (a rare herbalism node) and arcane-immune mana wraiths and other magic elementals in Netherstorm. I don't know how it compares to water but it's a pretty good living if you're not a mage.
Life primals come from felweed and from killing and herbalising nature elementals and giants in Zangarmarsh. They're a little more valuable than shadow I think.
Primal Might is only available by an alchemy transmute and takes one each of fire, air, earth, water, and mana. It's typically worth the value of the primals that go into it plus about 5-15 gold.