aaaquilterssupply.com carries wide quilt backing and they are the most reasonably priced that I've found. I'd use cotton backing. Hope you washed your flannel before making the quilt as it will shrink at a different rate than plain cotton. I would NOT wash it before backing with whatever you choose though!

Joanne's has broadcloth in wide widths that you could buy with a coupon. The color choices are limited though. Several have suggested a cream color, it comes in that color.

This really turned out pretty. If you want fabric as wide as the top, how about finding a flat sheet for the back?

oooh...that's pretty! I think I would go with a subtle multi-colored fabric. this web site lets you get free samples so that you can test several ideas & see what you're getting b4 buying...
http://www.backsidefabrics.com/sampleswatches.html