I got mine at Toys R Us a few weeks ago, but they seem to have sold out since. You can still order her online at Toys R Us, but I would recommend doing it through their ebay, since shipping cost seems to be cheaper through there than through their website.
The ebay auction is currently marked down half off, so with shipping, it's only about $10. Just do a search for "My Little Pony Twilight Sparkle Full Sculpt Ornament" and TRU's auction should pop up.