First, what is your intended purpose, concealment or as a hunting holster?
Concealment shoulder holsters usually don't work as well as you'd like for field and hunting use, and a hunting holster totally fails as a concealment holster.
The Bianchi X-15 is an old Bianchi design, possibly his first shoulder holster design.
It's been modified sometime in the past 20 years or so to add an extra retention strap arrangement to retain the gun in the holster.
The powerful wire spring in the front edge of the holster opening was always enough for good retention, and the extra strap and snap arrangement over complicates it, but does make it more secure.
The X-15 is available for the Ruger Redhawk.
Not many companies still make upright type shoulder holsters these days, most are the "Miami Vice" type horizontal shoulder holsters and certainly won't work fit the Redhawk.
A number of companies make hunting/field type shoulder holsters. These are not suitable as concealment holsters, usually are worn over the neck and shoulder and have straps across the chest to secure it from flopping around.
Here's just a few:
http://www.epsaddlery.com/c-25-shoulder-holsters.aspx
El Paso Saddlery is an old line company apparently related to the original 1890's company.
Their "1942 Tanker" is a replica of the USGI M3 Tanker shoulder holster and they make it for ANY gun. They also offer lining, carving, and hand stamping.
Quality is up there.
http://www.galcogunleather.com/shoulder-holster-systems_8_2.html
Galco Kodiak.
http://www.desantisholster.com/BLACK-MAMBA
DeSantis Black Mamba
http://www.unclemikes.com/holsters/sidekick-holsters/sidekick-vertical-shoulder-holsters
And
http://www.unclemikes.com/holsters/sidekick-holsters/sidekick-bandolier-holsters
Uncle Mike's are inexpensive nylon, which stands up in the woods well.
Other quality makers:
http://www.mernickleholsters.com/bm_chh.html
http://www.mountainhomeleather.com/shoulderholsters.htm