When compared to slicks, GMP's Can Am tire set, which is treaded consistent with the era that the tires represent, might be an appropriate place to start. These are first generation sets for GMP, so you have to find them aftermarket, such as on eBay.
I've got some hanging around in the "toy box". If you are interested, PM me.
I am not sure anyone sells rain tyres in 1/18. The thing to keep in mind is different era's of cars ran with different width of tyres and size. I wouldn't mind a set of rain tyres for my Senna 1993 MP4/8, I'll keep a look on this thread to see if something is found.