You could just go to Lowes or Home Depot and get 40 feet of Accoustic Research 12 AWG guage; probably about $1 a foot. Similar Monster cable at Radio Shack.
The closest thing for quality pre-terminated cables is at www.audioadvisor.com ---- Either Wireworld Luna III; $80 for a 20 foot pair with spades; or MIT Terminator 5 which is $65 for a 20 foot pair with gold pins. If the terminals are the Onkyo make it hard to use the spades, just cut off one side and it will be like pins. The MIT pins should work fine. If you go quickly to the AudioAdvisor clearance page you could get a 20 foot pair of MIT Terminator 4 for $87.50