Where can i buy gold coins?

[Raszak]

As above. You know, that gold coins that improve your drop chance from enemies

[Garza ///]

from lady in firelink after you give her old woman ashes i think but just only 3

[Cantyou]

Unbreakable patches sells unlimited amount of the Rusted Coin and for the Rusted Gold Coin it can be obtained by certain places in the game and a small amount is sold by the firelink shrine handmaiden

[lmaogg]

the old lady npc at beginning of dreg heap map sells a few.

[Raszak]

Thanks everyone

[Pyramid Bread]

Level luck to 99 and boom - permanent gold coin effect

[The Icecream Snowman]

A rusted coin gives +50 item find, gold give +100. I don't know exactly how much this improves your chances of finding stuff but the difference is fairly minor.

[Yal]

AFAIK item find = drop rate, so if you have 200 item find you find twice as many drops as normal. (You have 100 base + luck stat + buffs). Also, there's plenty of gear that boosts your drop rate, like the Serpent rings, the Symbol of Avarice, the Crystal Sage's Rapier... I'm not sure if the Shield of Want also boosts it.