The An-124 is not available new: Antonov stopped production in 2014. There are plans to restart production by the end of 2019.
Existing aircraft are owned by the Russian air force and by a few specialized freight companies. Neither are likely to sell their aircraft. So the An-124 is not available.
If you really wanted to buy An-124s, you'd have to restart the production line. In the past, Antonov tried to sell an An-124 version with Western avionics and engines, but the development cost required for those modifications was too high to make this an attractive offer.
Among Western companies, Russian aircraft are not popular. See the absence of Russian airliners in Western fleets. Antonov's attempt at a Westernized An-124 was intended to address Western mistrust in Russian engines and avionics.
The operating cost of an An-124 is probably higher than that of a B-747 (due to less efficient engines).
The extra internal volume might come in handy (IIRC cargo flights are often volume-limited) more than the extra payload capacity, but you'd have to find a way to utilize that volume using standard containers. You don't want to repack all your cargo if it flies one leg of its route with an An-124.
Because much of the extra internal volume is in height, you might have to build a second floor, which complicates loading. And IDK if the cargo hold is high enough to fit 2 layers of containers.