i've bought bolts & bolts of flannels on line over the years- i've been very happy with most- i purchase most from fabric.com- i read the descriptions- i purchase medium to heavy weight- prefer double sided flannel (both sides are brushed/soft---single sided is thinner, with only one side brushed)
many on line sites have a (swatch) program...you can order swatches of the fabrics you are interested in- then you have the swatches to refer to in the future.
benartex double sided flannel is my all time favorite flannel- the woolies flannels are next- (i think moda offers the woolies-but i could be mistaken)