100W+ of heat is a LOT of heat to get rid of, no matter what type of package it's in - it's all about temperature, in the end, and almost everything else ratings-wise is just BS. Honestly I wouldn't even try to get rid of that much heat without a massive fan, or water-cooling, etc. In this case, cutting down on the dissipation using some kind of (still very large) MOSFET and ideal-diode-controller IC like digsys suggested seems like the most realistic way to go.
What exactly are you doing with 100A of current? (Not a model train, I assume?) Any system that involves that much current is going to have a whole lot of copper, and even ignoring the diode, the rest of the 100A-related stuff is going to be expensive anyways. Maybe there's a whole different approach that would work better.
Also, for the future, here's how you can make some thermal estimates yourself with simple calculations (for a diode as an example):
1. Figure out your diode's power dissipation. This will be the forward current multiplied by the forward voltage at that current (look at the forward voltage curves in the datasheet, and choose some reasonable junction temperature like 70-100 C).
2. Find the "junction-to-case thermal resistance" (usually called Rth-JC) in the diode's datasheet. This will be in units of K/W, or C/W; it shows how many degrees hotter the junction will be than the case when dissipating 1W of power.
3. Find the thermal resistance rating in the heatsink's datasheet. A good heatsink datasheet will also show you different thermal resistance numbers for different amounts of airflow too, which will let you select a fan (based on the airflow) if you need one. Obviously if you're using a salvaged heatsink this will be much harder, but in that case find a heatsink with similar geometry on digikey, etc. and use its numbers as a ballpark estimate, then leave lots of headroom.
4. The junction temperature of the diode will now be (ambient air temperature) + (diode power)*(heatsink thermal resistance + diode thermal resistance). Pretty much all silicon devices top out at 150 C junction temperature, and for leaving headroom, getting decent lifetime, etc. I wouldn't recommend any more than 100 C at very most.