Here are some options that come close:
* "Xylophone xylophone" - This uses the "ks" sound of "x" in "xylophone."
* "X marks the spot, Xavier exclaimed" - While "x" sounds different in "Xavier," this uses the initial "x" sound to create a sense of alliteration.
* "Xeroxed xeroxes" - This uses the "z" sound of "x" in "Xerox."
It's worth noting that these examples aren't perfect alliterations, but they are the closest you can get with the letter "X."