Perhaps the most elusive of all tanking stats are the various magic resistances. You can't get them on any of the normal gear you strap to your various extremities, only on some trinkets. Some races have magic resistances as part of their racial bonus toolkits, but these are minimal and don't stack with any other resistance buffs. And speaking of which, two raid buffs -- Mark of the Wild and our own Blessing of Kings -- will increase all magic resistances by a set amount. Lastly, you can buff your resistances by a rather large amount with an elixir.
It fits that resistances are so rare considering how very, very powerful they are. Just the combination of a Prismatic Elixir and a raid buff can be worth about 28% magical damage reduction for fire, frost, and shadow damage, for example. Combine on top of the that the clicky effect from Mirror of Broken Images -- Image of Immortality -- which is worth 40.51% damage reduction by itself, and you're talking a serious percentage of magical damage just brushed off like dust on a sleeve.
A short history of magic resistance
In the early days of WoW, a key gear check for some raid fights was the requirement that everyone in the raid had to accumulate a set of gear with resistance stats for a specific type of magical damage. Molten Core was obviously the first, requiring raiders to grind out a complete fire resistance set so they could withstand the various fiery attacks that Ragnaros' minions would dish out.
This trend continued throughout vanilla WoW to fights like Princess Huhuran, for whom you needed melee players kitted out in nature resistance gear, to well into The Burning Crusade for fights like Hydross the Unstable. There, the encounter demanded one tank in frost resistance gear and one in nature resistance gear to correctly tank the fight. Black Temple had several fights that rewarded shadow resistance, and perhaps the grand swan song of the resistance set in The Burning Crusade was M'uru.