Yeah, tanking gear is nearly impossible to find pre-80 I'm finding.
I did mostly the same as the OP with DK. My wife and I started over on a new server so for my first toon I made a DK because I had never had one and figured it would be easier to establish an income than starting at lvl 1 (which, of course, it was.)
So for most of her early life, my DK was just a farm toon with herb/mining. Then I decided I liked playing her a lot so I screamed though Outland - HFP + part of Terrokar & Nagrand got her to 68. Did a lot of Howling Fjord and just kind of stalled out at 70. Then the patch came out with the LFG tool so I decided I wanted to tank with her. She had no "real" spec up to this point - mostly a jumble of frost/unholy that I had just sort of guessed at. So I checked EJ, specced her for the DW Frost tanking build (12/51/7 I think although only 7/51/7 so far) and then went looking for tanking gear . . .
There really is a sore lack of *tank* gear out there at these levels. I gave a bunch of cobalt to a BS guildy though and had the full cobalt tanking set made. My JC (and his) lack any tank gear til 77-ish but I made up a neck + ring with good dps stats and a gem slot that I popped +20 def gems into. Put +def enchants on cloak and . .I think chest? I've done nothing with her but queue random (instant groups every single time) from 70 to . .almost 74 now, and tank. At level 73 I'm sitting at, I think, 524 Def - not far off of being Heroic crit immune already.
Things to note:
-There is one tank piece that drops in the early instances; a ring in reg AN. Seems to drop pretty often.
-There is a set of tank gloves (better than cobalt ones) from one of the quests in AN (or maybe OK?) that gets turned in right outside.
-There are zero usable weapons/armor otherwise from the first batch of runs (UK, Nex, AN, OK)
I don't know exactly what the crit-immune number is at lower levels. But I started tanking at 70 sitting around 500-ish def and never seemed to have any trouble.
That cobalt set really seems to be the only viable option. Add in whatever enchants/JC pieces you can afford/obtain and have at it =) At level 72 or 73 you get the tanking runeforge which helps immensely. And from what I've been able to find so far, it looks like VH at 75(?) is the first actual decent drop chance offering a tanking cloak & legs.