There may be two types of sleepers: those who love flannel sheets and those who have yet to discover their love of flannel sheets. Maybe because I grew up in a big old house that was quite cold in the winter, I've always found flannel sheets to be a comforting antidote to crisp winter nights. Here are the best sources I've found for great flannel sheets:
A note about flannel sheets: don't bother looking for the thread count. Flannel is measured by weight (ounces of cotton/square yard of fabric) and you'll mostly see 5 oz or 6 oz sheets with 6 oz being denser, warmer and, usually more expensive. Also, especially with new flannel sheets, you will notice some build-up of lint in your dryer lint trap so be sure to empty it after drying flannel sheets.
L.L. Bean sells three types of flannel sheets: UltraSoft (5 oz, best value), Heritage (thickest and warmest) and Premium (6 oz), all made in Portugal. They have plenty of solids and plaids with a few traditional prints.
Coyuchi keeps it simple with just one kind of flannel sheet, but it's made from organic cotton and is weighted at 6 oz. It comes in four neutral solids which mix well with most duvets and comforters.
Garnet Hill built its business on European-style flannel bedding and have a wide selection of flannels for adults and kids. Of note is its line of Eileen Fisher flannels (shown top)and their Signature flannels which can be monogrammed.
The Company Store, in addition to many prints, carries a fairly large selection of both 5 oz. and 6 oz. solid flannel sheets as well as plenty of flannel duvets. Shown above is a nice modern, organic print called Lindsey (available as sheets or a duvet).
Target offers a selection of printed and solid flannel sheets under their Threshold name as well as some novelty prints. Many sets are under $30 and would make a good starter set.