I believe that any reasonably competent mechanic could be entrusted with replacing a set of spark plugs, including the dealer or even the Goodyear auto repair shops mentioned (They do routine maintainance in most of the US).
The reason to avoid the dealer would be an aversion to paying $363 to change sparkplugs costing at most $36 for all 12.
Dealers charge far higher hourly rates than nearly anyone else. Of course, sometimes this is merited (so far, not in the 62 years I have inhabited this planet), but I doubt that sparkplugs would be one of those cases.