I just shopped at JoAnn's with a coupon for muslin. Their premium "legacy" muslin seems to be good quality. There was quite a bit of variation in the different unbleached muslins; most seemed darker than what I wanted. Sometimes the tints are more to the yellow side than to the brown too.
I ended up getting some of the white "legacy". However, this brand came in different widths and I noticed variation in the shades of white among the different widths even within the same brand. This must be differences in dye lots.
Also found some Kona and other brands. Again, all of the unbleached muslins weren't quite the shade that I wanted. Most of the whites were too brilliantly white.