I got most of my disks via Craigslist from a former piano store owner who was unloading his leftover Disklavier inventory at a discount. That was a couple years ago and I think they're all gone now.
I think by far the best value for anyone looking to load up their piano with a ton of music relatively cheaply is this offering on eBay: 77 Floppy Discs All Styles Pianodisc or Disklavier. I'm not connected to the seller, but I did buy these disks and they're excellent. There's so much music in a variety of genres that I haven't even listened to half of it.
The only drawback is that the floppies are not labeled (in fact you have to put number labels on them, which are included). He does provide a spreadsheet listing the pieces on each floppy, but there are a lot of inaccuracies, especially in the order of play, so you're never quite sure what's on each and in what order. I'd like to figure out how to copy them to a hard drive with the track info listed so that I can go right to the piece I want.
If this is too much music or too high of a price, the seller has a variety of smaller packages based on individual classical composers.