As mentioned,, American Holly wood is about as close to real ivory you can get w/o actually getting ivory.
As for which grip frame,, fits which stocks,, well, remove the factory stocks,, and check the grip frame for markings. It'll tell you which ones are correct. However,,, realize that the newest Rugers will have the infernal,,, eeerrr internal lock assy that any stocks have to be hollowed out for.
I know,, as I made 4 sets of stocks for my 2 45 Colt New Vaqueros,, and my pair of Lipseys 44 spls.
If money is tight and you don't mine polymer grips, Ruger offers simulated ivory grips with Ruger medalitions for about $64. I really like mine. Any smooth grips will beat out the black cheesegraters that come standard.
IMHO, Holly looks great and does look a lot like ivory but it's a little too soft for my tastes. While I use my sixguns extensively, I am very easy on them and never abuse them. Still, my holly grips (three pairs) are dinged, gouged and scratched. My favorite ivory substitutes are TruIvory which is the closest you'll ever get to the real thing and antique ivory micarta.