If you are unable to find replacement cogs you could always turn the cog over and use the lesser worn part of the teeth. If the cogs have a spacer built in then you may have to grind it off and fit a loose spacer on the correct side. This seems quite possible to me (idea courtesy of my mate Steve who has done this, but I don't know which freewheel cassette he used). I shall stand aside while all the reasons for not doing this come flooding in....