The X-Men are part of the Marvel Comics lineup. They have their own unique place within the Marvel multiverse, with a dedicated fan base and a significant impact on the comic book world.
Well, popularity can vary. A might be more popular among long-time comic readers for its classic status. But X could be in the spotlight if it has a current movie or TV adaptation that generates a lot of buzz.
You could also try reaching out to comic book conventions or events. Sometimes, vendors there have rare or hard-to-find comics like Marvel x Fortnite. But it might take a bit of luck and patience to find exactly what you're looking for.