Target, walmart, and numerous other stores, but I would suggest you go to a home store like Lowes or The Home Depot since they will have a larger selection.
Target, Tj Maxx, and Ross all have both window treatments and shower curtains. I have gone between the three in order to get curtains to use as fabric for sewing projects as often as for actual interior decorating. There is a variety of color, texture, and thickness for such a reasonable price a I find it better than a fabric store.
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