Sirius optics have them... $365.. (
http://sirius-optics.com.au/ccd_image.htm)
If you already have the GPUSB, may as well go with an imaging source camera.. (although around here its not hard to sell a GPUSB
)
The DMK will yeild better planetary images than a DBK.. But I suppose it depends on if you are going to use it for planetary images... If you get a DMK with the UV filter (or if you already have a 1.25" UV/IR block), it will be easier to adjust focus on stars for guiding.