These unstudded all-season tyres from Michelin have a diameter of 18". They might be just the thing if you're looking for new all-season tyres.
Tirendo
Michelin Bib-Mousse Enduro (M15) ( 80/100 -21 )
Tyres-Guru
Michelin Bib-Mousse Enduro (M15) ( 80/100 -21 )
Demon Tweeks
Michelin Bib-Mousse - 90/90-21 & 80/100-21