Here you can search for tyres with the same tyre diameter but a different wheel size than your original tyres.
Please notice that you will also need new wheels which will fit your new tyres. The maximum allowed wheel diameter (as indicated in the registration certificate) can be increased by one inch without having to go through a modification inspection. If you want to switch to wheels with a smaller diameter, please make sure that there will be enough room for your car's brakes inside the new wheels.