[Skunkworks] Printer heads and cartridges

Hi All, Am looking for printing heads for a HP officejet pro 8500A plus. I have searched for them in vain. Could anyone point me in the right direction?I need the 940 black yellow and 940 magenta cyan.i would also appreciate if you can give me a quotation for the cartridges...940xl cyan and magenta. Regards, Brian.