Any of the DIY shops (like home depot or so) have this stuff. Get adapter from 1/2" to 3/4" and use lots of teflon tape fitting the stuff together. Maybe take the things on the photo to a plumbing supply and tell what you need. Just replaced mixer and shower last year. Quite a job to crawl back and forth in the boat, we had 35°C when I did that.
Made extra long shower hose so can rinse out spillings in the cockpit, and picked the best possible quality so hopefully replacement is not necessary soon.
If plumbers would just change to metrical system the world would be a better one.