Thats terrible news. It is highly unlikely you would find an OEM stock replacement, the best bet is I have a few times seen an 85L being sold as-is on ebay after someone tried to convert it and failed so everything ends up in pieces but usually not a broken rear element. I almost bought one last time that ended up selling for a few hundred dollars. The glue Canon used on these 85L's is the hardest I've ever dealt with in removing. The 85L would be a lens I would push people to consider having Jim Buchanan in California or Eddie Houston "The Lens Doctor" in England do the converting for on top of the professional cleaning and calibration they can do at the same time. Both can use my adapter kits if requested and they have in the past.
Ive got two 85L lens copies, both I paid around 800 dollars for but one is in slightly less nice condition. If you want to send me your broken lens I suppose I could use the rear element out of my second lens and perform the conversion for you. I'd be willing to do it for 500 dollars and you pay shipping both ways and I'll donate the adapter so you'd be getting your lens going again for just over 300. I'm doing this because my heart breaks hearing about this happening, the 85L is just so nice and deserves to live on and be used daily. I'll patch up my donor lens when I find another failed conversion at some point in time.
edit: I can also use your cracked element on my lens as a temporary measure, its surprising how much battle damage a lens can have without having that much of an effect on the IQ, in fact sometimes it adds character. The old lenses can celebrate their unique, glowing, sometimes distorted or imperfect but somehow magical optical qualities over todays neutral and flat IQ aiming designs meant to look like a high straight line on an MTF chart.