Feralas High wilderness, theres lots of ~lv44(?) ogres that drop mageweave and allmost no other players. Some ogres ~lv40 north of horde camp less drops but easier.
Tanaris ogres drop more mageqweave, BUT couse of "honor" system its a gankfest out there.
Those are good spots, but not incredibly helpful for lvl 40. I recommend Aeries Peak in the Hinterlands. Lvl 40-42 Trolls right near the Griffin spot (like 1 min ride on your mount - or less ) - they drop great loot (Got a Staff of Jordan there - woot!) and 2-5 mageweave a go.
with my lvl 50 shaman in around 1 hr I farmed approx. 70 pieces of magewave and in one of the troll camps there is always a chest.
so its a good grinding place! + can help with the donation of magewave quest = )
Another good place to farm for mageweave is the small camps just north of the northern road in the Swamp of Sorrows. The NPCs spawn quickly enough so that you won't have to spend your time waiting or running around.