There's two ways that I know of to get a "half cock conversion":
1) Jim Stroh at Alpha Precision does these, and has a page full of details on the process here:
http://www.alphaprecisioninc.com/revolver/halfcock.htm
Note that the safety is unaffected, and if desired, you can pop the loading gate open and it will function "just like any other Ruger New Model" as if you hadn't done the conversion at all.
I believe this actually gives you a "three click cock" versus four-click on a Colt. With Jim Stroh's mod, the trigger is further back in the triggerguard even with the hammer at full forward rest, effectively enlarging the triggerguard area for gloved use or similar.
Stroh charges $159 for the conversion. I don't know if that includes a basic trigger-feel action job or not; if it does, this is a pretty good value except that you have to add in gun shipping costs both ways, and the time delay with any custom gunsmithing.
2) Bolt in a complete conversion yourself with the Power Custom parts:
http://www.powercustom.com/revolver/parts_revolver.htm
Ron Powers has designed a complete hammer/trigger setup that gives you the half-cock conversion PLUS a quality action job, can be bolted in yourself (and by all acounts includes really good instructions) and as a bonus, you can choose between standard Vaquero-type parts or a Bisley hammer/trigger that will work in an unmodified Vaquero grip frame or a Bisley frame. Ruger's Bisley hammer/trigger needs minor alteration to work in a Vaquero grip frame.
Unless you have prior gunsmith abilities, I strongly recommend buying the hammer and trigger as a Powers-matched set. When you buy both, the sear face is pre-set for those two parts, it's very "drop in and go".
If you go this route, I recommend also adding the dirt-cheap freewheel pawl that will let the cylinder spin both ways with the loading gate open. This radically speeds reloading in some cases.
If it was me, I'd go the Powers route, esp. if you have eyes on a Bisley-bits conversion. The unpleasant shipping costs for a gun going both ways aren't present, you'll have it done a lot faster, you'll know a lot more about the insides of your Ruger, you'll have the spare parts in case of a breakdown, etc.
I've broken that Power Custom page out of it's frames, so to get to the ordering page goto:
http://www.powercustom.com
Brownell's also resells Power Custom parts, and you might get a better price that way.
Add $30 to the Powers costs, to get a set of proper Ruger SA gunsmithing screwdrivers from any of several sources.