I've seen boots like UT's at hot Topic, Demonia has some similar ones, but I think it may be easier just getting knee-high boots and attatching buckles to them. don't have to sew them on, if they're not that heavy, you could get a good thing of double-sided tape to attatch em.
I can't really hel you with the hat, because it's not a very common shape, but wat you CAN do is get a regular one with a full rim, cut some cardboard and put something like a stocking or leggings over it. (I found my hat at an antique shop; wrong color, but right shape, so yeah)
for the wig, what you NEED to keep in mind is that his hair, like Sephiroth's is SILVER. drives me absolutely NUTS when people get gray (much too dark) or white (much too light) for Undertaker cosplays. so what you can do is check around for long SILVER wigs (I think it was mentioned about 100 cm). if you can't find one, look for a Sephiroth wig, and just cut it to the needed length.
anyway, there's a sketch guide that will really help you:http://www.cosplayforum.com/viewtopic.php?f=1&t=22024
(just keep scrolling down). the jacket isn't really something to worry about, you can just find a long button-down dress, and make a separate collar. the overcoat is just loose-fitting, with REALLY long sleeves...probably can be made with a black bedsheet(?). don't forget the belt, which is just a gold chain with 4 memorial pendants, and don't foget the nails XD.
just keep in mind this guy is like 6'2, so just keep your own measurements in mind when you're doing this ;p
I hope this helps