If this is a repeated topic, just lock/delete it, but I couldn't find a direct answer of where to find a specific item.

Where can Level 4 key spheres be found?
I've yet to find any, and trying to find out what those so-claimed "Final Weapons" do is impossible.

For now though, it's about those pesky "Level 4 key spheres", who drops them? Where do I find more?

Level 4 Key Spheres - GameSpot.com (http://www.gamespot.com/final-fantasy-x/forum/level-4-key-spheres-41593517/)

good luck.

Wow someone else seems to be doing a playthrough aswell :D (if you are still coming back for replies then...) Firstly the legendary weapons do some random things, but they give your weapons abilities usually hard to aquire once you have found the crests and sigils for each. If you are near the 'end' of the game (the final aeon) you will start getting more level 1, 2 & 3 spheres, and you will have acquired one or 2 more lvl 4 spheres, apart from that, if you want to do it free roam from monsters go back to calm lands and bribe the chimera brains, i personally dont like the idea seeing as the suggested amount is over 9000 .... well actually its just under 200,000gil ... and i dont like throwing that money away