fabric.com has great sales- free shipping at $35 (i can always find $35 worth of fabric to buy!) and many different muslins- wide (90" & 108") regular sizes, bleached, natural---
they also have great sales- and a whole catagory of $1.98 fabrics.
i've been shopping with them for over 10 years- and have always been very happy- even when a mistake has happened (very rarely) customer service is great and they fix problems right away.