You can find chocolate cosmos at Longfield Gardens and I believe they offer discounts if you buy a lot at once. I would love to find some Middlemist camellia, but since there are only two in the world, that's out of the realm of possibilities. I chased it for ages because it thought it was so beautiful. Felt silly once I learned just how rare it was.
If you're into bonsai trees, then check out Dandy Farmer. You can find some rare, old bonsai trees at that site for sale. They ship anywhere in the US and are reliable. It will cost you, but that's the nature of collecting rare plants.