There's an episode of the Reading Glasses podcast that discusses this very topic - one of the co-hosts IS an author and she recommends the following:

- preorder the book if possible. This contributes to the "first week" sales of the author

- if you use your local library, you can request the library purchase certain titles for their collection for the community to check out

- Paperback purchases are completely ok!

- Even if you buy on Amazon/Kindle, the author still benefits from that in some way. BUT what also helps is making sure you leave a review of the book so others who may be purchasing from that place can see something positive