You can get a firmware at http://www.rpc1.org for the Pioneer DVR-109 now that supports -R DL. It is firmware v 1.40. I don't know if it's an official Pioneer firmware or not. It seems to work OK. I can't speak about the -R DL performance since I don't have any of those.
Sony also has it . It is the Sony dual layer tru - 800A dvd+/-r(w) burner ( I have one myself and so far it is running well ). As for the DVD-r dl blank medias , they can be purchased at this website : www.ritek.com & they have some good deals . check it out .