A five inch fitting is going to be tough, It's dinner time though, so the web search will have to be a bit later.
I'm guessing that a metal five inch is going to be easier to find though.
Thread below on "waste connection" deleted as tone and lanquage went awry. For Mike, I am sorry that on Sunday, a day of rest for many, that we were not able to post an immediate answer for you.
For the forum, he needed to connect a new drain to 5" cast iron. Is there a plastice 5 X 5 X 3 wye"
I won't go so far as to say he may have measured wrong, but the number of 5" cast iron lines I have seen in over 55 years is about zero.
Only one time have I had to deal with 5" cast iron. It was on a VERY old roof drain that got damaged during a huge commercial renovation project in downtown Cincinnati. I was able to get 5" no hub cast and bands, but it wasnt something the supply house had in stock. Give the plumbing supply houses a call....it might take a few days but they should be able to get what you need.
On a side note, I've dealt with alot more 5" clay pipe than 5" cast. NOT fun of course, but doable. An old timer set me on the right path when I was looking for a 5" clay X 4" plastic fernco. Gave me a 6" pl. X 4" pl one and told me to give it a try....worked like a charm. Might be an old trick for some folks in the field longer than me, but it did serve to help me think outside the box a bit.