meirman wrote in
:

Is it possible to get an 18MM spark plug for a lawn mower?

Of course it is. http://www.championsparkplugs.com/

Select "Search by Plug Construction" toward the bottom of the page.
The owner's manual or listing for the original plug will give you the
reach and heat range, look up the same or similar in an 18mm.

Note: Lawnmowers aren't real picky about heat range, just stay close to
the original.