You don't have to buy their books (although you could, and they would be endlessly grateful).
Here's what you can do to make a writer happy that won't cost you a dime:
If you've read one of their books, leave a review somewhere - Amazon, Goodreads, even just on your own social media.
Share the writer's social media posts. In a world full of books, word-of-mouth is still the best way for a reader to find their next read.
Libraries are how most writers learned to love books, and we don't mind library sales at all. Ask your local library to order your favorite format - most indie authors have books available in ebook and paper, and some have audiobooks as well (working on that - Songbird is done, A Wider World is in the works). The library buys the book, the writer earns royalties, and you - and other borrowers - get free books!