I want know where I can sell a big slab of ivory and how much it would be...

Do you have papers to prove how old it is... sales and posession of ivory harvested since the 1989 has been banned under international law.

Slab of ivory?

I thought all elephant ivory no matter how old is now banned (unless you are a country), unless it's a tiny fraction of a bigger item that isn't ivory - I think 5%.
Find people who advertise that they collect Asian antiques.

You have to know if it's Indian elephant, African elephant, or mammoth (from the grey lines and how they curve).
Mammoth is allowed.
Or if it's not ivory at all, but bone.
Ivory would not be a 'big slab.' How big?
Put a pin into a flame til very hot and immediately put the pin into the ivory. If absolutely nothing happens, it's ivory.

There is a lot of old legal stuff around... but from what I've read you need to prove its origin... meaning proving it existed before the ban went into effect.