I can't really help regarding sources but I have grown a couple of whites (although one was more of a cream). Aconitum.napellus album was a bit rubbish, tbh, A feeble plant which had to be staked or it fell over. I had much better luck with a, lycoctonum ssp. septentrionale 'Ivorine' which I still grow. A really robust and tough plant, which flowers fairly early for me - sometimes as early as mid May. It is a softer, creamier colour though. Have you tried 'Stainless Steel'? Another early bloomer but I do love the soft greyish blooms and cutleaf foliage.

I googled White Aconitum for sale and this is what came up. I hope some are near to you.

Digging Dog Nursery carries several white flowered types although not all are currently in stock. You can ask to be put on a waiting list for those, however. DD's plants are not huge but are well grown and healthy and shipped with care.

fromt he link below:

Aconitum, also known as aconite, monkshood, wolf's-bane, leopard's bane, mousebane, women's bane, devil's helmet, >>>> queen of poisons, <<<<< or blue rocket, is a genus of over 250 species of flowering plants ......

==>> now there is a useful common name.. lol ..

ken

https://en.wikipedia.org/wiki/Aconitum

I've placed an order with Digging Dog. As you noted, the whites are out of stock, but they did have some of these, Aconitum x cammarum ‘Stainless Steel’, so I ordered 3. They will make a good contrast with the ones I have. When I went to checkout I discovered they have a $70 minimum order and was forced to by more plants! The OUTRAGE! :o)

Thereupon the herb-master entered. 'Your lordship asked for kingsfoil, as the rustics name it,' he said; or athelas in the noble tongue, or to those who know somewhat of the Valinorean...'

'I do so,' said Aragorn, 'and I care not whether you say now asëa aranion or kingsfoil, so long as you have some.'

'Your pardon lord!' said the man. 'I see you are a lore-master, not merely a captain of war. But alas! sir, we do not keep this thing in the Houses of Healing, where only the gravely hurt or sick are tended. For it has no virtue that we know of, save perhaps to sweeten a fouled air, or to drive away some passing heaviness. Unless, of course, you give heed to rhymes of old days which women such as our good Ioreth still repeat without understanding.
When the black breath blows
and death's shadow grows
and all lights pass,
come athelas! come athelas!
Life to the dying
In the king's hand lying!

It is but a doggrel, I fear, garbled in the memory of old wives.2 Its meaning I leave to your judgement, if indeed it has any. But old folk still use an infusion of the herb for headaches.'

'Then in the name of the king, go and find some old man of less lore and more wisdom who keeps some in his house!' cried Gandalf.

@SeniorBalloon I can't really help you re sourcing (as I believe that I am in a different country than you) but if I may be so bold can I be the second person to suggest you consider Ivorine (as @rosaprimula mentioned in her post above). You are correct as it isn't pure white but rather, lets say, kind of...whiteish :).

I love seeing this aconitum bloom beautifully in JUNE (in my location).

I have had this single plant clump for several years. Here it is in June 2022:

(This spring blooming clump of aconitum is healthier and more vigorous than my two 'traditional' Fall blooming monkshood plants. I love Ivorine.)

Yep, me too, Rouge. A lovely thing. Both of my earlies definitely outperform the autumn bloomers (I have a Stainless Steel which must be almost 20 years old).

Both of my earlies definitely outperform the autumn bloomers

So I am not the only one to experience this.

I love Monkshood and I have tried several varieties but for the life of me I cant establish a vigorous Autumn flowering clump.

Yep, same. I get a lot of later self-seeded ones in the wood, but they always fall over and never make any sort of robust, self-supporting stand.

Very interesting. The clumps I have are late season bloomers and have made fairly good size clumps that mostly stand on their own. They do get some support from neighbors. One of them, when first purchased, said it needed shade or would tolerate shade, but it never did well and I moved it to a sunnier location 4 or 5 years ago, and now it does very well. The other has always been in the sunnier location and always done very well. And of course I don't know the name of either :o) The stainless steel appears to bloom mid summer, which is a bit earlier than the blues I have.

I will keep my eye out for an Ivorine.

Thanks,

SB

Indeed, I've read there are no true shade plants, just ones that will tolerate more shade then others. Although I'd put Impatiens Omieana in a true shade plant category as any sun at all makes them wilt.