How do do you generate B2B leads?

[domenikbrenner]

I was wondering how other sellers offer B2B leads in the thousands with extensive data as in contact persons, phone numbers etc. 

I assume they are signed up to Crunchbase or similar and just sell the results from there? The memberships are very expensive so it makes sense that a buyer might want to skip signing up just for a few thousand leads, but a seller could have many buyers so that it's profitable. Is that the case? If not, how else are they building their leads?

Just curious about how this works.