I just picked up a 1989 fzr400 and I'm finding that the 60 profile 18" rear tire is difficult to find. What are others doing about this lack of rubber.... has anyone tried a wider 70 profile 18" tire?

18" is a pain.
I think it's a case of fitting what you can get these days, or at least what what your tyre centre can get. Maybe give them a few weeks notice and see what they can come up with.

People look at 17" wheels as a more permanent solution but I cant really remember which bike. I think the XJR400 bolts in and maybe a suzuki GS something needs a bit more to fit.