So I spend alot of my winter in the rocky mountains. Which I have read in the Land Rover manual that you have to buy tire chains from Land Rover, I have a 2003 discovery II. Which I am not sure if some Nitto Terra Grapplers 255/55R18 will be fine or should I buy tire chains too? And does anyone have some good recommendations for tire chains?

This is my first 4wd vehicle, I have a 2wd truck before that chains were the only way to go in the mountains.

 

First, never believe anything Rover says, they can't help but lie.
Do a Google search for Bill Burke, he sells by far the best 4wheel chains on the market.

 

A good set of snow tires and a good 4x4 will do better than a good 4x4 with crap tires.

 

http://rudchain.com/index.html who makes the "Land Rover" chains.
A number of places sell them.

 

There's quite a few places to get chains, but personally I never use them. In NJ we don't have the worst winters in the world but we do get plenty of snow, and you can bet every time there's 8-12" on the ground you will see me driving the big white disco around before the plows, and never with chains. A good tire and a solid rig (what we all have ) will do way more than you think.