A /X is MEGA rare ! It ranks up with the /Y caution yellow lamps . As far as I know there is only 2 listed on this site ! As for finding one , they're probably located right next to the unicorns

Just keep your eyes open and closely watch places like eBay , ETC. The one HomeBrewLamps has was found in an old explosion proof fixture on eBay . That stupid fixture kept popping up in my eBay feed for months but I never noticed the picture of the /X lamp in the listing until AFTER HomeBrew bought it ! DOH !

AGAIN look closely !!!! Crazy things pop up where you least expect them !