Cannot find Ubuntu 18 as an instance option

This is the tool I use to find Ubuntu Official AMIs.
https://cloud-images.ubuntu.com/locator/
Set the filters at the bottom for AWS and version 18.04 and the region.

Be aware that 18.04 has reached the end of the standard support window and AMIs may not be available any longer. https://ubuntu.com/blog/18-04-end-of-standard-support

You can get it from AWS Marketplace at https://aws.amazon.com/marketplace/pp/prodview-pkjqrkcfgcaog#pdp-pricing