I usually see white, along with various shades of purple and lavender, in local nurseries, even though in general I don't live in an area with unusual choices in nurseries. So I would call local nurseries to see if they either carry them or can order one for you.

i would knock on a neighbors door.. and ask if i could dig out a sucker

ken

dig out a sucker

Assuming the neighbor's white one is not grafted onto another kind. You'll also want to do this when the plants are not in leaf.

Stark Bros is selling Mme Lemoine which is a double white. They are bareroot. Have never bought from them and no nothing about their quality.

Note: I am really not in lilac country, we have naturally acidic soils and they are shy bloomers for us.

Lilacs are in their full, gaudy, wonderful spectacle for us. Moscow Beauty is my favorite white. I think I'd start with an Internet search to locate area nurseries and then narrow it down to find the one you want. They're sold just about everywhere around here, but that may not be the case for you.