And then Rock Auto?
I would do Rock Auto first before those retailers, but then again Rock Auto is basically a drop shipper, but the vendors they drop ship from should be legit. Sometimes Autozone is cheaper once you use their 20% off coupon and sometimes Advance is cheaper.
But in your case, maybe consider FCP Euro. They carry your plugs in a similar price range and they do a lifetime warranty on all the parts they sell, you just have to return the old plugs.