If you want at least one copy of each weapon in the game you can race for 3 Dark Matters and 4 Adamantines. Barring that you can race for Elixirs from the Manasvin Cup and Myrmidon Derby races in the Guardian Class races. Of course, you would be better off retiring each time you win one of those races instead of using a ton of shuffle items. And if you're not into THAT you can always do the secret races. Each of them gives you an adornment the first time you beat them.
Adamantine you can get from the Long Gui in Archylte Steppes. Dark Matters I believe you can only get at the races, but I didn't bother looking elsewhere for them anyways since I didn't mind racing for them.