I use Hershey's Special Dark Cocoa Powder in many of my recipes; it contains 1 net carb compared to the 2 net carbs many cocoa powders contain. Hershey's Natural Unsweetened Cocoa Powder also contains 1 net carb.
I use Swiss Miss Diet Hot Cocoa in some recipes; the new label calls it Swiss Miss Light. This is not the same as Swiss Miss No Sugar Added.
You may be able to find these and other cocoa powders in your local store. You can also order them online. Check prices; it may be cheaper to order in bulk.
Here are some links to check out.
* Low Carb Chocolate Sources *
Making your own chocolate treats is a great way to control the ingredients. There are other healthy sources for chocolate, too.
I advise you to stay away from chocolates sweetened with maltitol, sorbitol, etc. These may be sugar-free, but they can cause blood sugar spikes and "gastric distress."
There are some better alternatives:
Gift Candy Ideas
My ideas using recipes from Laura Dolson from About.com's Low Carb Diets
LC Foods Dark Chocolate Chips
Lily's Sweets Dark Chocolate Premium Baking Chips
Low Carb Specialties ChocoPerfection Bars