I love Moda flannel. MSQC has a good selection of it.
https://www.missouriquiltco.com/shop...Moda%2BFabrics
Be forewarned, while really good at shipping out precuts, they are abysmally slow in fulfilling yardage orders. My order took 2 weeks before they shipped and that was only because I contacted them. Then the shipping took another full week so if you want your fabric sooner you may need to find another source.
I have also had good luck with flannel made by clothworks, northcott, Henry glass and Bernatex. Thousands of bolts carries all those brands and while searching their site, it appears they have a wideback flannel by Moda. TOB definitely has the best prices. Here is a link to the moda wide back I found. Disclaimer, I have never used Moda wideback flannel so can't attest that the quality is the same as their standard width flannel.
https://www.thousandsofbolts.com/ind...00&ParentCat=5