Try ebay .. I found a great pair of Ariat tall boots that retail for like $350 and I got them for like $200 with shipping... they didn't have a zipper in them so I took them to the cobbler and he put one in, works great! :)
I know a ton of people who have bought Ariat Challenge boots (the zip ups that are like $350) and I know several of them who've had the zippers break on them. Within 3 or 4 months of owning them. And apparently Ariat does NOT have any kind of warrenty or return policy on broken zippers! If I had to replace my tall boots I'd take the new ones straight to a boot repair shop and replace the zipper for a more heavy duty one. Don't get me wrong, almost everything I own is ariat and most of the stuff is better then a lot of other brands out there, but the quickly decreasing quality level is making me really irate (and broke).