There used to be a book.

An enormous green bible, published by the Directory of Social Change, called the Directory of Grant Making Trusts. If you were serious about finding funders in the early 2000s, you knew this book. You probably borrowed it from your local voluntary action organisation because buying your own copy was out of the question. It cost a fortune then. It still does. A new edition still lands every year, for anyone who can stomach the price.
You sat with it, worked through it, dog-eared pages. It was heavy. It was slow. And it was the single best starting point available.