I’d have a look at Barton elite silicon, they do all sorts of colours and sizes including 18mm.
https://www.bartonwatchbands.com/col...=8813106364470
I’ve got one and find in really comfortable. They are also extremely good value so you can experiment with minimal worries over cost. They sell them on Amazon.co.uk but in my experience it’s cheaper to buy direct. They charge some minimal amount for worldwide shipping, $2?, and if you buy individual straps they come in under the UK customs threshold for duty which is something like £17?