Ink is available in a variety of types, not all of which are suitable for fine-art drawing. Make sure you choose a pigmented lightfast ink, not a dye-based illustrator's ink which fades over time. I prefer a basic 'Indian Ink' which dries waterproof, flows well and doesn't tend to clog. Indian ink can be thinned with distilled water (tap water will make it separate), but I prefer watercolor for washes. Many companies make Indian ink, most of which will be acceptable for drawing. The 'buy direct' links in this article are to online art supplier affiliate, Blick Art Materials.
1. Winsor and Newton Black Indian Ink
This is your standard Indian Ink or Encre de Chine, made from carbon black, with a Shellac medium which provides its water resistance and glossy sheen. (This can also make it a bit of a pain to wash up). The manufacturer says it has a 'bluish undertone' when thinned but I found it rather neutral. My bottle of W&N Indian Ink has the standard screw cap, but this one-ounce bottle at Blick comes with a built-in eyedropper - great for adding controlled amounts to a wash.
2. Winsor and Newton Black Liquid Indian Ink
This rather oddly named ink is a non-waterproof ink made from Chinese ink sticks of Lamp Black. It has a noticeable brown undertone which I really like - it's warm and appealing. A more watery consistency than true Indian ink, the pen doesn't hold as much, and it bleeds more on fibrous paper. For these reasons, it's a good idea to try this ink out on a sample of your paper to test its performance and get used to its slightly different handling. If ordering through this affiliate link, be sure to find 'Drawing Ink... LIQUID Indian' on the order form.
3. Dr. Ph. Martin's Bombay India Inks
Need color? Try these are gorgeous inks. These are intense, pigmented color inks, with excellent lightfastness (less so for the Violet and Magenta). They flow well from pen or brush and can be used anywhere you'd use watercolor - ideal for all art and craft work. They are relatively quick drying so be sure to wash up promptly after use.
5. Yasutomo Sumi Ink Sticks
Sumi-e ink sticks are lovely to use, and quite inexpensive. You'll find sets with stones, brushes and sticks at many art shops as well as Asian importers. The lamp or carbon black used in these sticks offers a lovely, velvety black. Because you're mixing the ink manually, it can be tricky to get a consistent proportion of pigment into your ink, so you'll want to test your ink before you use it. It can be nice to actually emphasise the natural variations that are possible with this ink, for a soft and organic quality in the work. These inks are intended for use with brushes, rather than dip pens.
I don't personally use fountain pens much - I prefer dip pens. One thing I do know about fountain pens is that you mustn't use shellac-based waterproof inks in them, as they clog the pen - sometimes permanently. Buy specialist fountain pen inks that are designed to flow smoothly from the cartridge. Here's an excellent page from the 'Pendemonium' website which has loads of advice on inks, with a list and comments on many brands and colors.