How do I find or retrieve purchased songs whose files cannot be found?

Andy,

The first place to look is on your computer (including the Trash/Recycle) and any copies or backups you have made.

If you are still using the same Apple ID, are in the same country, and the content is still available, you may be able to redownload fresh copies at no charge. Follow the steps in here:

https://support.apple.com/en-us/HT201272