I'd like to replace the front sight blade on my Ruger RSM with a white bead. I see that NECG makes one, but does anybody offer a slide in model with an ivory bead?

Yes,
Brownells has them with a 3/32 bead and they are very nice indeed and I think that NECG, is now offering the same.

I bought mine from Brownells and it was actually made by NECG. It went on a #1 45-70 and was MUCH easier to see at dawn/dusk than the factory bead.

Actually, the same can be accomplished by painting the rear surface of the factory bead with fluorescent white paint. If you file the rear surface of the factory bead a little so that it angles upward (before applying the paint), it catches even more light.

Live well

Any good gunsmith can replace the existing bead with an ivory or micarta ivory bead....they are just stuck in there...make a new 3/32 bead and glass it in..Maybe cheaper to buy a new one in many cases.