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.
You can use the Slumbering Dragoncrest Ring or the sorcery Hush to make no sound while moving, so you won't have to walk instead of running anymore. If you have no ring slot left for the Ring of Fog, you can alternatively use the Hidden Body sorcery as well. If you get detected and want to escape enemies, you could also try to use the Chameleon spell after running away to stop them from following you.
This forum post also suggests that the weight of torso armor affects noise and detection by enemies. I remember that armor made a difference in Demon's Souls, but I am not sure how much it really affects noise in Dark Souls. It would make sense though that light armor is less noisy.
Generally, having both the Ring of Fog and the Slumbering Dragoncrest Ring equipped should make sneaking around much easier.
Best Answer
Big Hat Logan sells a copy as well, and is initially found in a cage within Sen's Fortress. After getting past the second platform containing swinging blade traps, you'll quickly come across a sleeping snake guard leaning against a wall. Big Hat Logan is located in a room hidden behind this wall, only accessible by breaking the wall.
To break the wall you need to redirect the metal balls you see throughout the fortress. The mechanism to do this is located after the second room with the metal balls coming at you. You want to redirect the mechanism to the hallway opposite the exit which will simply drop the balls off the game map.
In order to unlock the cage he is located in you need one of the following:
After you free him from the cage he'll return to Firelink Shrine, where he'll start selling spells. He'll stay there until you put the Lordvessel on the Firelink Altar, at which point he'll relocate to The Duke's Archives. Make you sure buy his copy of Great Soul Arrow before he moves, since he won't become a merchant again until after you've gone through a large portion of The Duke's Archives.