A nice national frame store is Aaron Bros--see if there's one not too far away from you.
I have never found archival-quality Pottery Barn-type frames at Michaels and ACMoore.
So I usually get my stuff online--Dick Blick, Exposures, ArtCityFrame are some of the places I've found.
Also check out, online, Light Impressions and University Products.
In my Northern Virginia area, the Pearl Art chain and a very small art store with a few locations, Plaza Art, frequently have some.
Nielsen Bainbridge is a manufacturer that makes what you want, too. Go to their online site to see if it can direct you to vendors in your area (otherwise Dick Blick online, as well as a number of other online sites, has them).
The places I've mentioned certainly stock ready-made sizes. Dunno if they can do custom sizes, although when I want to have matching frames, I just buy a slightly larger ready-made frame and go to a framing shop to custom-cut a plain white archival mat. (but then again, I like some of my mats cut off-center, or "weighted," so they look OK to me.)

try framefit.com

I use American Frame Service (www.americanframe.com). They have both wood and metal frames and will cut them to size. They will also cut a single free mat in a variety of colors with each frame.

Try Exposures (exposuresonline.com). Lots of photo frames, books and other photo archival supplies.

Thanks for the recommendations -- are there any B&M places that sell better quality frames? Are inexpensive frames like the ones from Target, Kohls, etc okay for long term storage of photos?