Well, finding proper disks these days is difficult. They aren't physically quite the same as "modern" 1.44 floppies, due to how the oxides are laid out. However if you can find unformatted 1.44 disks, and format them as 400k, they work well enough. You can also always format 800k disks for 400k, but those are also not easy to find.

I believe MacDrone is correct about the 800k drive, I don't think its compatible with the old 64k ROM Mac 128's. It will freeze up because it doesn't understand what to do with it. You'd likely need an external 400k drive, and good luck getting one of those at a good price.