Almost all makers of holsters will an offering for a 5900 series S&W. I would probably make a few decisions as to what features you do and don't want, what materials you want it made of, thumb strap or not, OWB or IWB, etc.
Most of my herd rides in Kramer #3 IWB or Milt Sparks VM-2's...they are NOT a cheap holster by any means.
A 5906-5904 is not a lightweight gun so make sure that you are using a very sturdy belt. If you can fold it in half vertically it is not up to the job.
Another great solution for many on this forum is to call Ray (lobogunleather.com) and have him make what you want. Great product, with great pricing, and a fabulous man to deal with.
Randy