Here's a few I can recommend after extensive use of their sheaths (no specific order):
Mike Sastre (River City Sheaths)
GrinderMcGee
Azwelke
Okuden- rarely hear about them anymore:/
Others that I have not yet had the opportunity to try out:
Dave Brown (BuyBrown)
Lots more but that's just off the top of my head. I can honestly say that ALL these gents are good guys and great to work with. Mike is nearly always backlogged and last I checked had a six-month wait for a sheath, but it's worth it, he specializes in concealble IWB Concealex for BIG bowies. Al Welke and Grinder both were about two week waits, but you will need to contact them for specifics. Okuden has their own site and I really don't know much else about them. Hope that helps.