Buy a shield. No warrior should be past level 20 without a shield that he can pull out for instances.
Generally, tanks are hard to come by. Even harder to come by than healers. Since tanking is one of the hardest things in the game to do (I'd argue CCing while DPSing on a hunter is harder) you have even less people willing to do it. Which is why a warrior (who has more "Oh smeg" buttons than a pally or a druid) is usually wanted for a tank.
Also consider what else you have to bring to a group, besides tanking...you can't bring CC, you can't bring heals, and generally those are what people look for besides a tank. Yes, you can off-tank, but polymorph and traps dont require heals. So unless you end up with ANOTHER tank in the group (i.e. another warrior, or a better geared druid/pally) you are probably going to be expected to tank.
In raids it's different, as usually you have the designated MT and OT.