I buy Roc lon 403 and 404 ( these numbers are on the end of the bolts) , one is bleached the other is natural . It is pre - shrunk , but there is still a bit of shrinkage but not nearly as much as the other muslins. It is perm - press as much as an all cotton fabric can be. I can tell you from experience , the other muslins that are not perm press .... well you will spend LOTS of time ironing after you shrink them.
Some quilt shops carry them. Jo anns used to carry it about 10 years ago .. but I have only seen the lower quality there for years.
http://www.fabricdepot.com/index.php...age&pageid=193