It's not too bad a job putting on a new unit, few hours tops. I could help find one if you posted the model and year of the 90HP mercury so I could check around for you.
I sent you a PM and replied to another thread with some search results from google.
I also have a good friend who is a master marine mechanic and mercury dealer. I have dropped the lower unit on mine and changed out the impeller and cup, as well as repaired the screen for the water intake. Biggest hassle is getting the right tools and getting the old unit off. If it hasn't been serviced in a while the shaft can get stuck, worst case they have to be cut off but that's unusual.