I have wondered the same. I know he was close with the owners of Mountain Oaks Farm so perhaps you can contact them. I know them only from sales at farmers markets...

I agree his death was a very great loss to California horticulture. There just aren't that many people around with that kind of passion for plants anymore. I ordered a lot of 6 mature seed grown A. belladonna from him to test in various parts of my garden to hopefully select a form hardy enough to grow in my zone 7 east coast climate. He enjoyed speaking to me on the phone about my project. Only one has survived, including the polar vortex winters, although it doesn't bloom every year. When it does it is spectacular and the fragrance of those is enthralling - no other monocot matches it I think, including lilies. I wanted seed grown ones for the genetic variability. (I did not become familiar with this plant in CA because I've never been there in late summer. Seeing hundreds of them blooming around Lago Maggiore is what got my heart set on them)

Two possible leads in the mail order space: Telos Rare Bulbs in Humboldt County. Diana is a superb nurserywoman and if she doesn't grow something like that herself, she might know where to find it. Likewise, Cistus Nursery in Portland has a very large collection of Amaryllidaceae on-site, only a fraction of which makes it into each year's mail order offerings.

Interestingly I believe I have seen one of the east coast catalog merchants, possibly John Scheeper's, once offered an all-white x Amarygia, intended for greenhouse cultivation by most of their customers though they obviously do fine in CA, but don't recall ever seeing a US nursery offer all white A. belladonna other than the Bulb Baron. (People w/older memories will remember, "The Bulb Man", another CA mail order merchant, but he seemed to focus on Iridaceae). I have now flowered the x Amarine recently offered by Brent and Becky's. It's pretty, but entirely lacking in Amaryllis fragrance!