I went to the ARS website and searched for any white azalea 3' tall in 10 years, hardy to -15F and white. It came up with 1 possibility, a Weston.

Weston's Innocence, which is deciduous

Going up to 4' in 10 years, it came up with 4, all deciduous.

Daviesi, Fragrant Star, Snowbird, and Sylphides. Fragrant Star and Sylphides are the hardiest.

Going down to 2' in 10 years, it came up with 2, both evergreen.

Helen Curtis, and Marie's Choice

Here is a link that might be useful: American Rhododendron Society Azalea Finder

I have a white Chionoides Rhodedendron, which doesn't grow to high, 4 feet at the most but it grows wide, it's a pretty dense evergreen variety and looks stunning when it's in bloom. I purchased mine at a garden center in Lunenburg but they do have them at Home Depot, I saw them there a week ago. Mine hasn't bloomed yet but has huge buds ready to pop very soon.

Linda

My favorite of the low, white blooming evergreen azaleas is 'Everest', although it may not be hardy enough for your zone (-5F). I'd consider 'Snowbird' as a reasonable alternative. It is deciduous but produces a profusion of small, sweetly scented flowers and has a very compact habit compared to most deciduous azaleas.

Thanks so much for all your suggestions! I've noticed that many of the nurseries around here now sell very few cultivars, and they seem to often sell the same ones, so it's time for a web search. I'll check out all the options and see what I can buy without increasing my carbon footprint (i.e. not having it come from Washington State...)

Not an azalea, but some of the dwarf Mountain Laurels might work. I have 'Elf' and it has stayed very petite.

There were several white varieties in Greer's EVERGREEN AZALEA section (including Gumpo.)

I love my Rhododendron 'Clipinense' which is white, low growing and early.