I'd start by checking to see who is doing good work locally, look in the phone book. While there probably isn't a listing for sheath makers a lot of saddle makers, holster makers, etc. take on smaller projects like sheaths to help keep busy, you can also check with local hobby supply places they may be able to point you to someone doing leather work. There are a lot of people out there doing good work, but it saves a lot of potential hassles to have someone local that you can get to know them and see their work. It saves a lot of time to be able to walk in with the knife and talk face to face, make a sketch, pick hardware, color, etc. rather than playing phone tag, e-mailing back and forth and faxing. Not to mention the shipping problems that can pop up. Not to discourage you from lifter4Him, Sandy Morrissey, or any of the others, but I've found (from both sides of the fence) that it is often less hassle and just as good a product to stay local if you can.
Todd