You have to remove the valve cover, but that's pretty easy. The spark plug seals are kinda like part of the valve cover gasket, they fit around the sparkplug hole. A valvecover gasket 'set' often includes these, but check to make sure.

If it continues to leak, it may be from the lower seals. For these you have to remove all the rocker assemblies & camshaft bearing caps. That makes it a much bigger job than the valve cover gasket.