Garden Crossings carries it.

Well they don't offer it for sale right now but another nice white that caught my eye was the Clematis Candida. Is it the largest white blooming clematis? I read one source said the flowers are 9 inches across and another that said 5. That's quite a difference.

Many clematis nurseries have sold much of last year's stock and haven't yet put up their list for next growing season, so you may do better finding specific varieties if you wait until January or February.

The source I use most often to get info on clematis is Clematis on the Web. I find flower color in photos isn't always accurate, but at least it isn't enhanced as in many nursery photos, and I find the information unbiased unlike some nurseries' info. You can type just the name (or part of the name which is best on a multiword name since the format doesn't like spaces) or any other single or group of factors such as height and flower color to get a list of possible plants. Just leave blank any part of the form you don't know or don't care. They say Candida's flowers are 7"-8 1/4".

As far as size of Candida's blooms, there could be several reasons for the disagreement. Many of the type 2 (early large-flowered hybrids) bloom different sizes depending on whether they are from the season's first blooming (previous year's growth) or second blooming (this season't growth.) This also may effect whether a blossom is double vs. single. There also seems to be some confusion as to this particular plant vs. several other plants, and having several different plants that might potentially be called Candida may also be part of the issue. (There is an International Clematis Society article on Candida linked from the CotW page on this plant that you can read.)

You also may want to add your general location to your profile so that folks know a bit more about what your gardening situation is. For much of the US this is at or past the best time to be planting clematis. Here's how to add that:

Go to Your Houzz in upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone.

If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures.

Then return to the top of the page and click Done Editing.