if they dont have it, best buy should, it should be less then 30. i've used them in the past, it beats the old school way of cutting off the factory plug and splicing. and this way you can put the stock radio back with zero hassles.
these work well if your using all the factory wires, speakers and such.
if you going with a amp and new speaker wires, you can also just run your own power and grounds and let the plug hang back there. <<<no need for the adapter if you go this route.