I'm currently levelling leatherworking, and instead of playing ridiculous prices for the mats, I've been farming them. Though I can't find any place to reliably farm Heavy Leather. I've looked through a lot of sites, and the majority of them say to farm Yeti's in Alterac Mountains, or Raptors in Dustwallow. Neither of these places are dropping much heavy leather at all, mostly thin and medium.
I always look at the "gathered from" tab, or in this case the "skinned from" tab. Looks like you want mobs between level 35 and 40, top of the list looks to be Dustwallow Marsh, with all the various packs of raptors, dragonkin, crocs, etc. Might be a good place to try.