Why would you want to buy one ignition item at one place and it's cousin at another?
Buy a matched set. If you don't want to pay dealer price, search the web site for oem parts providers. You could patronize the parts house here at the top of your screen - FASTLANE.
I wouldn't put NAPA, Auto-Zone or for that matter, any non-OEM ign. parts on my MB. You get what you pay for.