what kind of gun is the soldier holding in the modern warfare 2 cover? Where can I find it?

I have good news and bad news for you.

Good news is that the weapon he is holding on the cover of Modern Warfare 2 has a name. It's the Barett WA2000.

  The WA2000 was designed in the 1970s. The bullpup design was chosen because it would allow a standard length (for a sniper rifle) barrel to be used whilst the overall length would be shorter, thus making the rifle easier to transport, than a conventional rifle.

The WA2000 used the PV4 night vision sniper scope.

The .300 Winchester Magnum round was chosen as the primary caliber because of its long range accuracy and its consistency at all ranges. The entire rifle is designed around the barrel. The WA2000 fires from a closed bolt and uses a bolt with seven locking lugs. The rifle features a six round magazine capacity. Only 176 total rifles were ever produced in three variants: one chambered for the standard .300 Winchester Magnum round, another chambered for the popular 7.62x51mm NATO round, and the final version for the 7.5x55mm Swiss round. The final retail cost for a rifle in 1988 was in the range of $9,000 to $12,500.

Bad news. It hasn't been released to the public for purchase since 1988 and may be illegal in some states. Looks like you will have to be content with just using it in game.