I don't play 19's so I can't judge that guide... But I checked out their f2p guides and the enchants there were completely wrong (i.e. telling f2p's to use p2p enchants lol). You can try out the 19 guide but I'd be wary.
Check out this awesome guide by Shft:
http://www.twinkinfo.com/forums/f148/comprehensive-guide-farming-chests-19-24-boes-46486/
Basically, Magician's Mantle drops from either certain mobs or chests. The drop chance is low in both cases but you'll have better chances of getting it through chests. Although, RNG is RNG and some lucky bastards end up with the item on their first shot. :0
The link above is basically a guide to chest-farming. The preface kinda explains the way looting works and which chests will drop items of what iLevel. Hence, explaining why items like MM drop from the chests that they do. There's also SS albums that help explain the location of the chests. It's a whole lot easier to find them with actual pictures to guide you. On top of that, there's some other info and tips too.
The guide should hopefully clear up your questions about MM. :0
