How do you lot buy backlinks?

Honest answer: carefully, or not at all. Paying for links breaks Google's guidelines, and the cheap packages are mostly links on sites that exist only to sell links, which is exactly what gets ignored or penalised. On a brand new site I'd put that money into something people would link to naturally, or into digital PR / guest posts where the placement is a real editorial page. Fewer links, but ones that stick.