Where can I find large downhill ski boots? Size 18 mens?

You would have to go to a store and ask them if they can order some from the companies. I know they make up to size 15, but I have no clue about bigger than that. They won't fit in rental bindings because the rental plates don't adjust that big. You would definitely be looking at a pretty big bill at the end of it though...

Well...snowboard and ski boots are different so you would need a pair of each if you wanted to do both sports.

As for the size.....sheesh! 18, huh? I think this is going to mean a visit to a professional bootfitter on your part. Mondo size 31 (US men's 13) is about what you will find in stock at any ski shop. Certail places might be able to call in a special order. There are custom bootmakers like Dale Boots whi might be able to help.

As for the mounting....that shouldn't be a big issue. You might just need a longer ski with a thicker center area so the ski will be properly balanced.

There is on sure-fire way to get some ski boots that will fit your big feet, but it's probably not the option you will be looking for...but here it is anyway.

Long before the invention of plastic boots and step-in bindings people wore leather boots and a cable binding. While the old style cable binding has died off, people still use a 3-pin 75mm Nordic Norm binding and leather boots with a duckbill on the front. These can be hand made, not cheaply, but probably still easier to get than a custom size plastic boot. The drawback to using a leather boot and a 3-pin binding is that you ave to learn how to Telemark. I still see guys at Mad River Glen skiing in leather telemark boots so I know they are on the market and a good cobbler can still probably make a set.