Jay, I used to inventory a good selection of oval frames. At some point, I realized that I personally didn't care for the look of an oval mat inside an oval frame. Often, they look like a bulls-eye target to me. Also, because of size limitations, it's not always possible to have the oval window proportional to the oval frame. Just to my eye, that looks bizarre.
More often, if the image requires or benefits from an oval mat window, I'll make a rectangular - or, occasionally, an octagon - frame.
Normally, now, if I use an oval, it's with spacers and no mat. BTW, spacers in an oval frame are a special challenge. I usually cut a 3/16" mat from rag board and hide it under the frame rabbet.
You've heard Inline Ovals mentioned many times on The Grumble. Also, check out Presto. They have a very large selection of ovals, circles, fan frames, etc.
I had some ovals that warped so badly before we used them, that they seemed determined to fold themselves in half - another reason I don't keep any sitting around any more. They weren't Presto or Inline, though.