After you beat Riku mode, there's nothing to do really, unless you wanna get all cards and level up to 100. And as for the weapons....
What exactly are you asking in your first question? If you're asking what to do after beating the game with Riku then there is nothing left to do because that game is done.
As for the weapons: One Winged Angel is acquired from objects in Traverse town, Diamond Dust is acquired from Castle Oblivion. Both are very rare, however.
To get Ultima Weapon you have to Have beaten Reverse/Rebirth and must make a save room in both games right before the final boss. Then make a calm bounty in the game with Sora and you will get the Ultima Weapon.
Actually, Ultima Weapon comes out randomly from jumping on objects and hitting objects, you can't get it from the Moogles. It's all luck and chance.
And I got One-Winged Angel in Castle Oblivion, the last world. just go to the moogles, they will eventually give u a pack that has One-Winged Angel, once you get it, it comes out frequently.
And Diamond, same thing as One-Winged Angel.
So basically, you can get One-Winged Angel and Diamond Dust from anywhere.
Oh, and if you want to get the Sora cover back, then just save the game in Sora mode, and if you want Riku's, just save the game in Riku mode(reverse/rebirth).