Where can I farm Titanite Chunk?

Where you would farm it depends on where you are in the game.

The absolute best place is just before the last area:

In the Kiln of the First Flame, there are a series of five Black Knights just before you fight the final boss. The first and third ones have a 100% drop rate for Titianite Chunks, and respawn when you rest at a fire. This is the best place in the entire game to farm Chunks, but unfortunately the only thing after it is the final boss and a NG+

The next best place to farm Titianite Chunks is in Anor Londo (you'll get there after ringing both bells and clearing Sen's Fortress). Just before the boss fight in that area, there are two special kinds of Sentinel (big giant stone-looking dudes, their weaker cousins inhabit the Darkroot Garden) known as Royal Sentinels. Here's a video of someone fighting them; it's kinda boring because he tries to lure them up the stairs but you should be able to get an idea of what they look like.

This is the only place in the game where that particular style of Sentinel spawns, and they're the only kind that drop Titianite Chunks; they have a 1/4 chance of dropping that particular phat lewt, so there's a 50% chance of getting one every time you kill the pair. The closest bonfire is, unfortunately, on the other side of the mist wall so you're probably only going to start grinding the Royal Sentinels after you've made it through.

The third-best place to farm Chunks is in the New Londo Ruins, after you've done something about the water; in the normal course of the game, you'll do this at some point after Anor Londo. In the newly-exposed area, Darkwraiths will spawn.

These guys are pretty vicious and don't drop Titianite Chunks very often, but some people prefer them over the Royal Sentinels simply because it's more fun to fight them than it is to fight the Sentinels; more of them spawn per run, and they're a much more interesting opponent. Here's a video of someone going through the process of farming the Darkwraiths. Although he scores two Chunks out of a total of about five Darkwraiths killed, keep in mind that he came pretty close to dying several times - and if you're not careful enough in that area, the Darkwraiths will gang up on you which is not fun times.