You can ususally find darker brown eggs for sale at the store marked 'free range' or just plain 'ol 'brown eggs'.
If you are looking for the super dark, chocolatey brown eggs- those are usually laid by french black copper marans, and since they are highly valued birds (in the case of pure-bred, non-hatchery stock) the eggs are usually pretty pricey. Like up to 30 or 40 dollars a dozen pricey. I think most people sell them as hatching eggs (meaning fertile).
I dont know where to get those super dark colored eggs just for etching- maybe put out an ad on CL, maybe a local person will have them?