I have seen Monster Energy Drinks at Sully’s Sips on the Wonder. Pretty sure I’ve seen them on the other ships as well. Look for them at the pool deck locations where you can buy a Dole Whip or drinks in the souvenir sippers. They may be available at other locations as well, but that is where I’m sure I have seen them. I don’t recall what flavors they were. The selection was limited. Maybe only one or two varieties.
Pretty sure I have seen Red Bull as well. That may have been in the Lounges.