Sally's sells them. I usually get them much cheaper off ebay for my wigs in bulk.
Also I sewed my crown on for my Red Queen wig with thread the same colour as my wig. I created 4 small holes spaced out equally at the bottom of the crown to sew through and tack stitch through the wig. For a larger crown I go with 5 of 6 wholes to sew down to the wig.