What are the materials that you don't currently have (or perhaps only have a sample of) that you'd love to be able have in your palette to blend with, but just can't find (at all or in a quantity or at a price you can deal with)?
To start us off here are two items on my wish list, one impossible and one I aim to solve:
The impossible one is violet flower absolute (hasn't been made commercially since about 1902 I gather) but my goodness I'd love to be able make a fragrance with some.
The other is geosmin - I have a little but no reliable, affordable source for more so I'm reluctant to make a product with it - I'm working on it though.
So what are yours?