there are lots of online quilt/fabric shops that carry wide flannel for backings.
fabric.com is one-
widebackings.com is another....there are lots more....just do a search for wide flannel.
you can certainly use regular cottons on the back of your flannel quilt if you choose- but flannels are available.
visit the fabshophop- and look through their list of shops (between 130 & 180 of them- you will find lots of resources for great flannels- and everything else. they also have a fabric search link to help you narrow your search.
flannel sheets can be used-but usually start pilling fairly quickly
i love the benartex flannels - and the moda woolies flannels are wonderful too