I know that some of the Nordic makers like Gustaffson, Pearson, and similar use mammoth ivory recovered from the North Sea and it has a very different look to it, from the contact with water I suppose as it fossilized.
It may be true that the rare colors like deep blue and green aren't as easy to find as they once were, but there are lots of knives out there with very good ivory, its kind of like when you hear the common lament: "Oh, you just don't see stag like this anymore", well, yes, I see it all the time actually, any time someone says: "You just don't see stag like this anymore..."

What I hear is: "This is
very good stag".
The point being I guess, it's still out there, and I don't see the supply of very good ivory drying up so quickly, though maybe the very best of it, ie. vivid blue/green, is in shorter supply.
I admit, I am a big sucker for really choice fossil ivory, mammoth or walrus, I have more than 1 knife I bought probably in large part because I was so taken with the ivory.