For those who may be interested, Sansung has released an update for the messages app that will allow you once again to set custom notification tones. It should be available for you now or very shortly in galaxy apps.
I don't know about the Play Store. I relayed all the information from the Samsung website.
However, if you're in a hurry, a quick web search for Samsung messages ver. 18.104.22.168 should find it for you.
I just checked and it is available on APKMirror