I don't know of anywhere it's sold specifically, but here's a list from Grechen's closet that details online places that sell Karen Walker. Perhaps you could email them if you don't see that bag online.
i saw these in a teen vogue earlier this year...i went hunting for one, but think i could only find one boutique that carried them and gave up. they're really adorable bags!! let me know if you find a place to get them from!