Feather Craft,Cabela's, Ehillie Angler Supply House, Hook&Hackle.. about any good fly shop should carry Dee's Quill's... A lot of different companies marketing them these days....
Feather-Craft also sells Turkey Rounds....and has instructions for use for very fast dry fly bodies... Not Quill but fast and durable body material...
Ron's suggestion of Biot's is a good one to for some patterns...
Quill bodies will always be my favorite material...Just makes the best representation of the waxy segmented mayfly body,That I've seen to date.
Wish ya great fishing,Bill