Another chiming in and saying "build it yourself" - use a TonyMacx86 Golden Build, there's a list of components on their website that work flawlessly with OS X, and detailed threads from most of the users as to how they got each of their builds working. I can vouch for the ease of some of TonyMac's builds. I've worked with difficult builds, such as laptops and unsupported boards, and I also have my own that was a recommended build. I can safely say that anything TonyMac recommends in the Buyers Guide works without a hassle. Quite honestly, installing OS X on my custom desktop was easier than installing Windows on my Mac Pro.
Also recommend Micro Center. Great deals on CPU bundles. I also know for a fact they stock compatible Gigabyte boards.