I typically go with custom kydex, and make my self my own leather sheaths.
AzWelke makes a great sheath, and no need to send in your knife if he already has the model. I have only owned one of his, but was very very impressed! Perfect fit, and my ideal retention.
BuyBrown, and MashedCat also typically have the blade in house for many blades.
I have owned all three, and been very very pleased with all. (all mine right now are BB).
Another one great maker I have tried was Okuden. On a passarround SHSH from Mulder. That sheath was heavy duty, and very well put together. The retention was very positive.
I have only owned one specopps sheath, and it was not a perfect fit for the knife I had (a custom Bill Siegle). It was secure, and attractive, and had good attachment options. I liked that my leatherman fit in the pouch along with some fire making bits like my ferro rod and striker and a few bits and ends!