Don't bother with a garden center....there are far too many different cultivars of clematis for anyone other than an expert to ID anything beyond the most obvious/basic. A clematis specialty nursery might help but they are few and far between :-))

One of the easiest ways to narrow thing down......and you may never confirm an actual ID.......is to Google with key words like "large white flowered hybrid" and then opt for images. Two things you want to keep in mind for comparison is the coloring of the center boss of stamens and the number of tepals and how extended they are in your photo (almost displaying a petiole).

https://www.brushwoodnursery.com/products/clematis-duchess-of-edinburgh

Hi Chey !! Whatever it is , that pic is beautiful!! I thought of this clematis when I saw it .

Chey, What a stunner! I searched around and found ‘First Love’, but it has more wrinkly tepals than yours. You might search around this website though.

http://www.clematisontheweb.org/new-clemdetail.cfm?dbkey=3963

The Duchess of Edinburgh photos on Clematison the Web doesn’t look like yours, but I just saw the link Lilyfinch posted and that looks like a dead ringer!

Look at Arctic Queen too. Mine often has white/green mixed on twisted leaves close to the bloom.