Although you can often format HD floppies as DD, it's preferable to find some 720K disks from the PC world and use those instead. These disks are the same, media-wise, as 800K Mac floppies. Just format them as Mac disks, and you're good to go.

Office supply stores no longer seem to stock 720K media (at least, not around here), but I see that many online sources continue to sell them (at widely varying prices; shop around).