Check their website first for contact information. If there’s a contact form, you can try to contact them that way. Next, reach out via social media, or through their agent or publisher (their contact info will sometimes be listed on the website). Don’t be afraid to share with them what you loved about their book when you make the ask, and remember that if you’re shooting for “pie in the sky” authors you may be disappointed. Many authors get asked ALL THE TIME, so make two lists—one for your reaches and one for those authors who are more reasonable so that you don’t reach your pub date without any blurbs lined up.