Firstly, you do not, in fact, need to be at 540 defense for heroic 5 man dungeons at level 80. The defense cap for these dungeons in 535 currently. That said, you do need to be at 540 in order to tank Raids. What these defense 'caps' ensure is that you aren't critically hit while tanking, causing what would likely be your death and an ensuing wipe.
My suggestion is the same I make when a guildie asks about getting ready to tank. Start with the normal instances for ToC, FoS, PoS and maybe HoR (this one is a real bugger most days) and get yourself some decent gear. Once you're at the defense cap for heroics, you can move on to them and work your way towards the 540 you need for raiding. So for now, guildies and the Dungeon Finder can be your best friends.
What this accomplishes is not only getting you egared, but experience in tanking situations. Gear i the simple part of the whole puzzle, experience is what tends to be much more valuable and much harder to come by.