No, you cannot write AmigaOS floppies on your old Macs. The Amiga floppies are 880kb, where the old Mac floppies are 800kb.
You can find the floppies you need at eBay, AmiBay, and a few other places.
Downloading the images from the Net is not legal either, as just like any MacOS (was MacOS7 freely released?), the IP has not been released as freeware. Any help toward getting a download cannot be discussed on these forums as there are strict rules against any kind of piracy here.
There is a device, a floppy controller, that can be installed in a PCI slot of a Windows PC, called the "Catweasle" that can read and write Amiga disk files (adf's). I don't know if the Catweasle can work in a Mac, but do a search on it and you should find all the info you need.
