One option would be to switch your tyres for something with a higher aspect ratio to give a taller sidewall. You can use websites such as www.blackcircles.com to search for your existing tyre by size, and then search again for tyres with a 50 or 55 aspect ratio which should fit on your existing wheels.

Alternatively you could swap your 18-inch wheels for 17- or even 16-inch wheels, as wheels of this size are available on other versions of the Yaris Cross. This would also mean a new set of tyres, but would give a greater increase in ride comfort.