You can buy them on the underground or some people believe that you can get them if you have connections with a high page rank website that has a television network. These kinds of links will sky rocket your traffic overnight. If you want proof then read my Hub http://whoisbid.hubpages.com/hub/How-to-make-a-pop... and go to the evidence I provide by finding out about someone called AJ who has written about it and given all the proof you need. I am talking about extremely powerful links from very high page rank sites and not those social network bookmark links which don't really do anything for you because the search engines know that they were designed for that purpose and most likely they hold no weight. Don't waste your time with those kinds of links.
The other way is to make friends with people (bloggers) and start a link ring (be very careful that you don't actually make a ring though because you will be leaving link wheel footprints. You can also slave away for several years and build up lots of sites and blogs with all kinds of information and then when they get popular you can use them to link to other stuff (but don't overdo it)
The answer is either money/connections or simply years of hard work.