I have purchased a lot of flannel and my favorite company is moda. There marble flannel is over the top. I have purchased from Hancocks and there's is good too. Usually is the price is to good to be true, the fabric is no to good.
I have also purchased from www.fabric.com and was pleased. Sometimes they lack quanity if your looking for a large amount. I love the heavy white, I made a design wall with it.