Ron Loc makes several grades of muslin they have a very good premium muslin. Many sellers do not put the thread count on the listings... they just say high quality or premium .. well that not exactly a guarentee you are getting the best or highest quality.
I have had great success with Ron Loc premium , permant press, pre-shrunk. Just a note the "pre- shrunk" still does shrink just a bit. ..but the quality is high so I stay with it. The manufactures number is Ron Loc 404 or 403 , depending on if you want bleached or unbleached.
I purchase each by the bolt, these have more than standard quilting cottons on a bolt. I think the last time I purchased one it had 50 yards per bolt.
Not all muslin is PFD ( prepared for dying). Ron Loc does have a muslin PFD , its number 408
I used to split bolts with family , before I moved too far to make it convienient.
Here is a link to all the muslin fabrics from Ron Loc
http://www.roc-lon.com/products/muslins.html