What everyone else said. Tank spots are notoriously tough to find, although they do occasionally pop up; usually OT spots where someone is impatient with the gear they're getting or wants more control (tanks, by the nature of the role, tend to be prima donnas). If a decent guild's MT quits/retires/moves, usually the first OT will just assume the MT role. Your best bet for tanking roles, especially if you're not pre-BiS (with all the gold pissed down the sink that that entails for overpriced BoEs), is to find a guild that's not raiding yet but still building their roster. The problem with that is that it means spending months waiting to see if the GM can pull enough people to actually make raiding happen. A lot of guilds with designs on raiding end up being leveling guilds, as they can't get 60's to stick around long enough to get a raid group together. It's far easier to get into raid groups as DPS or heals, as you need more of those than you do for tanking (prior to Naxx, anyway)
I got lucky; I was pulled into a guild in my early 50's that was setting up a second raid group, so they already had the population to retain people and I didn't have to seek out a tank spot, it was dropped into my lap by the GM, who took a liking to me. The down side is that I'm tanking for an NA guild and I work (in the US) at a strip club, so I can only make half of the raid nights consistently; that's left me as the OT2 because it's a waste of gear to throw tanking gear at someone that's not there for every raid. Means my chances of ever getting my hands on a Thunderfury are slim to none (a man can hope, though!).