I've never seen it for sale up here (Bay Area). However, I have seen Greyia radlkoferi at Annie's Annuals, but not in the last couple years. They misspell it "Greyia radkoferi", so it might not come up in a search. If Annie's ever sells it again and they make it available by mail, you can also special order it from nurseries that carry their plants. It's possible they grow it in small enough numbers that it is only be available at their retail nursery. In the past, they sold beautiful plants, and the price was reasonable.

I've seen Greyia radlkoferi at Strybing Arboretum, but have never seen it for sale. I saw a Greyia at UC Santa Cruz--noticed it in bloom from a distance. I did not find a tag, so I don't know if it's G. sutherlandii or G. radlkoferi. I don't know the difference between these two. I have not seen Greyia for sale at their gift shop. I rarely make it to their sales.

The attached picture is one I took of the UCSC plant.

Here is a link that might be useful: Greyia radlkoferi at Annie's Annuals