Where to buy Diff/tranny case oil

Hey guys, after seeing what my local shop wanted to charge to change the diff/case oil i'm gonna do it myself so any of you guys know/use an online resource to buy the oil being that it ain't cheap or do you just use brick and mortars?
also I remember reading threads going back and forth about synthetic V regular, and frankly I'm not driving in friggin alaska and I'm no doing a ton of off road so unless there's a spec or other specific reason to spend the extra on syn then i don't see a prob with the regular.
Thanks in advance guys.