The only place I know that you can buy legal Kickstart files is Colanto with their AmigaForever CD, or download. But their Kickstart files are encrypted and need their ROM key, so I don't know if they would work for you.
I think the solution is to run their AmigaForever program on your PC and then use any Amiga program that saves the running Kickstart to a file. Then it can be used without the key file.
