Khaire!
Well, I made my Corinthian crest the same way I've done Roman ones,
http://www.larp.com/legioxx/crests.html
http://www.larp.com/hoplite/corinsd.jpg
http://www.larp.com/hoplite/crstft.jpg
(Yeah, I need a few more good photos, particularly of the process!)
First off, the crest block should NOT be taller at the front than at the back! That's an error started by Deepeeka in India resulting from a misinterpretation of a photo of a reconstruction. It should be in the 1-1/4" range for the whole length.
I've never made one out of leather, only wood. It's a matter of separating the hair into tufts and gluing them into holes in the wood. You might do the same thing with layered leather, perhaps? Or use a wood block and cover it with leather? If it's for SCA use, I can certainly understand not using wood, though, since it will break.
About half a pound is what I use for a full-length crest, though the crests from Albion Armorers use 2 to 3 times that much. VERY nice, if a little heavy on the head! One pound should be plenty. I cut the hair to about 12 inches, then fold the tufts in the middle so that they are about 6 inches tall. Seems to work fine. On my Greek crest I left the hair of the rearmost few rows the full 24 inches, to form the "tail".
I haven't shopped for horsehair for a while, but I'm surprised you can't find white. Leather suppliers such as Tandy or Leather Unlimited ought to carry it, either by the ounce or in 1-lb bundles. Of course, if you're looking for WHITE white rather than the natural blond, that could be a problem. You might try Tails and Tack, tailsandtack AT yahoo DOT com. As I recall their prices were a little high, but they'll do custom dyeing so they might have whiter whites.
Sing out if you need more help! It's a tedious job, by the way.
Matthew