There are often local shops or guys who make great doors for about the same price as the mail order. Check your local building supply companies but not the box stores. You need to find where the contrators go. Maybe even call a few contractors and ask. They know who makes what. Almost all towns of even small size have at least one guy making doors in his small shop.
If you are in or near a larger town check under hardwood and/or lumber-hardwood in the yellow pages. Most medium and larger town have these types of stores that make anything but thier cost is a little more. With this said their quality is typically second to none. If you could see the white oak stairs I had custom made from wood in stock in their shop you would drool. Very nice. I could have used Maple and in fact they have truck loads of maple in stock at any time including figured maples. If you have the $$$ they have the wood ... oak, maple, zebra, cocobola, blood, etc doesn't matter. If they don't have it they get it.
With all of this said the sites listed are probably very good and there are probably more. I've never used them since I've used local guys with super great success and similar or cheaper $.
Hope this helps