Newbie Question. Direct Advertising in high PR websites? How?

I don’t know this is the right place to post mu question

I need back link from good PR website (PR5, PR6, PR7). I would like to place my website’s direct Ad on high PR website, either (Banner Ad or Text Ad). As i am to enhance my service, I thought advertising on high PR website helps in increasing both traffic and PR. My website has a PR of 3. I would appreciate if you provide me the procedures to get those backlinks and also the rates bit elaborately (i.e monthly, yearly, permanent ).