Ghiradelli and others make a 100% unsweetened chocolate.
Cocoa nibs can be found at Whole Foods, in the vitamin section. Any health food store would probably have them, too.
I like to make a flax brownie recipe (Miracle Brownies) that I found online, using unsweetened chocolate, cocoa powder, sucralose, and erythritol (sugar alcohol which doesn't cause stomach distress).
If you want a sweetened chocolate with no carbs, most of them are sweetened with Malitol, a sugar alcohol that can cause gastric distress (stomach aches, etc). However, I recommend a product called "Choco Perfection" made by Low Carb Specialities, which is sweetened with Sucralose (splenda) and not with Malitol. You can only buy it via mail order. The bars are expensive (almost 3 bucks each, and they are sold by the dozen) however I think they might actually be worth it! The dark chocolate is what you want (they make a milk chocolate version too). I also buy their 'baking chips' lately, because it's easier to exercise portion control with the chips... once you unwrap one of the bars, it's damn near impossible not to eat the whole thing. Doing that is not hard on your waistline but it's hard on your wallet.