To everybody struggling to find tanks, try running dungeons without one. All-dps runs are totally ok for a lot of 5-mans. Two or three mages with polymorph and frost nova can keep quite a few trash mobs locked down. Hunter pets and voidwalkers can act as poor man’s substitutes for tanks. Bring 2 or 3 of them, and they can take turns sacrificing their pets to buy time before players themselves are next in line to die. Spread out a bunch of ranged dps in a circle around a boss so he has to waste time running to each of you in turn, then cycle through aggro drops like ice block, feign death and vanish to make a boss spend as much time as possible uselessly running to people while ranged burns him down. Any random 60 resto druid can tank BRD arena bosses, and probably a lot of other things too. Some bosses can simply be burned down without anyone tanking at all.
Remember you don’t always need to do a full clear. Even if the last few bosses in a dungeon need a tank, maybe the first few don’t. Go through your BiS list, or if you’re leveling take a look at any drop table, and notice how many items you want that aren’t from a final boss. If it takes you 40 minutes to find a tank for an hour long full clear, you sometimes might be better off grabbing 4 dps and a healer and spending that hour and 40 minutes running the first 2 or 3 easy bosses in a dungeon several times instead. Some bosses can be skipped to very easily. You’re probably not going to clear SM Cathedral without a tank, but Hydrospawn in Dire Maul for example can be reached without stealth, by killing only a single solo trash mob.
For bosses behind a lot of trash, try skipping them anyway. Many classes have mechanics that allow parties to skip trash. Rogues and druids can stealth. Hunters can kite packs and feign death. Mages can blink past things. Anybody can use an invisisibility potion. Try suicide-running through a dungeon with somebody fast or durable in the lead, then have a paladin cast divine intervention on a priest to ressurect the party in some safe corner in the middle of the dungeon. Stack your party correctly and you may be able to skip trash without a tank to get to specific bosses you want.
Some specific classes group can stack themselves so effectively that the tank/healer/dps trinity doesn’t even matter anymore Try a run with five mages. 3000+ dps can very easily burn through mobs before they can even reach you, and when they do, 5x frost nova and 5x ice block thenm cold snap all over again, makes it really hard to wipe. You don’t really need anyone to be “the” tank or "the "healer when you have a group composed exclusively of feral druids and ret paladins. Try a mix of 5 exclusively hunters and warlocks and see if your 5 pets can tank long enough for your to kill it without any tank or healer required.
If tanks are hard to find, try running without one.