If you're in the US, you can get them at any of the major online office supply store such as Staples (staples.com), OfficeMax (officemax.com) and OfficeDepot (officedepot.com). They only cost minimally more than HD disks.
Or you can purchase them in bulk with old software on them at Atari dealers such as B&C.
Just don't buy them for $1 per used disk like some fools are trying to do!