I had the same problem, you CAN get it in act 2 on a quest I think - but I found it in act 3 after I already had much better stuff, and I was like "so THATS where it was!". I can't remember where now though..
the wiki says.."Found in a grave in the Mountain Graveyard just beyond Sundermount Passage during Act 3."
..oh I was wrong. It appeared on Merrils quest now I think about it - a bit pointless as I already had most of the champions gear by that point!
ok...not in Sundermount (at least in act 2)...not in Mountaintop Graveyard either (other cool stuff though)... this is bad design imo...my rogue has full set of equivilent armor (forgot name) while still in act 2