I want to know how websites such as https://www.intelius.com or
http://www.peoplefinders.com able to get the datas of people. i mean
where do they extract it from ? and how. do they get it for free ? if
they are paying, whom do they pay to and how much.

i want to run similar website so any helpful info is really appreciated :)

Most of these sites use purley public records such as phone records
and electorial rolls , court attendance records, register of births
deaths marriages , property records, magazine subscriptions.
Or they pay data miners for it (postal order companies, mobile phone
companies, mail orders, book clubs)
Some countries also make avialable census data , but a period of time
after the collection date

if its public record, where can i get it ?  or do they buy from a
company or something? how