One thing I buy occasionally is a carton of chicken livers. I know they're not carb free (still only 0.02 g carbs/100 g serving), but the price is excellent (I think the last time I bought them, they were only $1.29 or $1.39 for a pound of livers at a local SE Pa store.)

I usually fry them in butter, and eat a couple of them every day for a few days. I crave them every now and then - not sure why, but I think it might be due to their high folate content - 100 g of chicken livers has 588 mcg of folate (147% RDA). That's almost 3 times the amount in 100 g of spinach.