Eight key flute in D is the most trad option and Hobgoblin (you can do mail order) have loads both new and second hand. Just check the hole spaces but the stretch is pretty similar to a classical metal boehm flute in most cases. Many years ago I switched from a metal flute to a traditional one. It takes some practice but I enjoy playing both. I use the D flute for most things but when it gets into awkward keys I use the metal one cos it's easier to get b flats etc.

Book wise there's a great tutor book and tape by Fintan Vallely (again available thru hobgoblin. This teaches you breathing techniques and tricks suchh as rolling and cranning and cutting and you can listen and hear how it's done.

Best of luck

Sarah