Hey, if you're at all handy (or know someone who is

), you can also sew together two sheets - sew up the top, sides, and a bit around the bottom - you'd want the bottom to have an opening about 1/3 -1/2 the width of the down thingie. My ikea duvet cover has no closures on it - just a opening in the middle of the bottom. I'd lay the comforter out on the sheets first, to make sure it would fit, and allow a bit extra for seams, but since it would all be straight seams, and the edges would be finished already, it'd be easy.
Otherwise, target has some organic stuff - the more expensive stuff online is from a more reputable company - the cheap stuff I dunno about. Gaiam has an outlet section worth checking (lots of pink!!).