Anglo-Saxon Ink

We usually imagine the Anglo-Saxons as invaders, from across the North Sea. But I wanted to explore the world of learning and ask a simple question – what can we learn about the Anglo-Saxons from ink? I know only a few people – mostly churchmen – were literate – but great store was placed on books. Where did the ink come from – after all you couldn’t just pop down the shops and buy a bottle of ink or a pen, could you?