I've used Moda flannel that was (and still is) wonderful, but it was regular width that I pieced. Last week, I ordered both regular flannel (Timeless Treasure) and a wide back (Wilmington) from Hancock's of Paducah. DH reports that the pkg arrived today, but I won't see it until at least Saturday, as I am out of town babysitting.
Not sure what they have left, but the Timeless Treasure plaids were $3.99! I'm hoping to be able to piece them so that the seam isn't too often obvious, but at that price, felt they were definitely worth a try. If the seam is unsightly, well... They are for my own quilt, and it's the back, right?
The wideback from Wilmington was also on sale, $11.99/yd! Hopefully after washing (flannel being pretty much all I ever wash), it'll look and feel nice. It's for a quilt for our DD.
I wouldn't recommend flannels from big box stores. Not only do they shrink even more, they pill like crazy.