I have a molded K-frame leather holster that fits my Security Six like a glove. So most holsters should be more or less interchangeable.
After I got mine last year I ordered a new ejector rod for it straight from Ruger (I marred mine up trying to remove it), so they do still have some parts laying around.
And the internals are incredibly simple so any competent gunsmith shouldn't have much trouble with them. You can actually field strip these guns in a matter of minutes and all the tools are part of the gun (rim of a .357 cartridge to take off the grips, a small pin inside the pin that holds on the grips to take out the main spring, and the piece that holds the main spring acts as a pry-bar to remove the trigger/grip assembly from the frame).
Unfortunately grips seem to be almost non-existent. Last time I checked Ruger had their large target grips on clearance for about $10.
Get the gun, and figure out the details as you go. You won't regret it.
"And I'm tellin' you son, well it ain't no fun, staring straight down a .44"