In Team Galactic building in Eterna City, in the chamber to the right after you've found the bicycle guy with Jupiter.
Chat with our AI personalities
in iron island
The Magmarizer can be found within Route 214.
in rout 228
When looking at the item, it does say "a Pokemon loves this item". It is a hint to give a Pokemon that item. Well that Pokemon is a rhydon. Give a rhydon that item to hold. Trade it and see what happens...
You may need to find the vs. seeker in Pokemon Diamond, Pearl and Platinum. You can get this item on route 207.