I agree with Ricky, never replace a single disk as you'll end up with a mismatched pair. Replace both disks at once.
The ones in the link Tim gives above are the "flower pattern" ones, which are the latest and lightest centres. You could go with the straight spoke ones as there's not that much difference between the two weight wise, but obviously lighter = better.
Brembo are fitted to Ducatis because they are the best - just look at any MotoGP bike to see what they use. Admittedly you're not buying the same kit as Valentino Rossi has on his Yamaha, but the knowledge they have amounts to a lot. While it's perfectly acceptable to replace the oem no-make disks from the latest YZXRRGSPVFF with some pretty Galfer petal disks, they may still not perform as well as good ol' Brembo.
That's my thoughts, but it's your wallet and your choice
