when I've needed black or dark brown in the past I've bought ready made coloured fondant from my local cake supply shop. Otherwise, cake supply shops will have the paste (don't use liquid - the paste gives a much better result) in every colour imaginable so you can make your own black from just a basic white fondant. Make sure you wear gloves though when you're handling it or you'll be scrubbing your hands for days to get rid of the colour.

Not sure what website you're looking at, but I've seen quite a few of the cakes now that are the end result of one called Cake to the Rescue. Looks like a fantastic way to get exactly what you need. Unless you do decorative cakes often you would spend more than that on ingredients/decorations if you bought it all individually, so to my mind they're actually a very reasonable price.