Does anybody know an easy way to get coins?

If you've done the Stones of Barenziah quest, you'll never run out of money ever again.

Explain??

If you do that quest, you get a perk called Prowler's Profit that causes you to find lots of rare gems like flawless diamonds and emeralds whenever you open a new box in a dungeon. Since you literally find these gems all over the place in storage in dungeons, you can literally find thousands of gold in gems as a result of this perk in one dungeon. I've literally been over-encumbered because I have too many gems on me. No more money troubles for me

I almost have all of the stones. I only like a few more. Does that only apply to brand new chests that have never been opened or does it also apply to a dungeon/chest that has been reset after a certain amount of days??

New chests and likely those that have reset

Where can I find this quest?

I think you pretty much have to finish the thieves guild questline to get it.