I have seen some DD disks for sale on ebay before, they may still be there. Usually in packs of 10-50 or so.
I have used HD disks with my amiga DD drive without any problems although I hear there are some properties with the HD disks that they should be used only with HD drives to avoid possible data errors. (i.e. don't use for important data you don't want to lose unless you have a HD drive)