My very favorite Black ink is this brand by Yasutomo. This is Traditional Chinese Sumi Ink and I like to buy it in Ultra Black because it dries a little bit shiny. I prefer Sumi Ink over FW Acrylic or even Higgin's Eternal because it is less likely to bleed.
This comes in a large container, so it will need to be transferred into smaller artist cups or an ink well.
I bought this ink at Paper and Ink Arts. I have also seen it in the store at Hobby Lobby.