I have seen variegated forms of lily of the valley growing in some of the gardens I have visited in the past and in every single patch of it there is some reversion back to the plain green leaves. The variegated leaves on this plant are not completely stable, but the normal green leafed plants can probably be easily removed to preserve the variegated plants.

The sources for this plant online all look like they have sold out of it for the season, I don't see White Flower Farm even offering it for sale this year. I would never purchase anything from Brecks, have you looked up their reputation on the Garden watchdog site? It isn't great.

https://davesgarden.com/products/gwd/c/183/

kitasei - I found variegated lily of the valley for sale and it hasn't sold out. The price isn't bad but the shipping is a little expensive because they are on the west coast. I have gotten plants from this nursery and they are always strong plants of good quality.

https://farreachesfarm.com/collections/shop-all/products/convallaria-majalis-albostriata?variant=47458165326106 

Far Reaches Farms is one of the best online nurseries on the planet!! Slowly taking the place of the revered Heronswood for rare or unusual plants.

I grew the variegated LOV in a former garden. The good news is that it does not spread as aggressively as its solid green cousin. The bad news is that it is very slow to establish and spread at all.