for marketing yeah. there are many ad networks you can buy them from. its a mixed experience sometimes good, sometimes bad.
Of course there is this possibility too. Buying backlinks can mean a certain investment in your site. But buying a couple of backlinks won't rank your page, nope it's not that easy. You need to buy contextual relevant backlinks from high PR pages and i might add they need to be in large numbers. Try to avoid getting too many backlinks to your site over night because you will be blamed for spamming. If you have lots of money (backlinks are expensive a PR 7 link could cost you around 70$ a month) you could go and buy backlinks but if you have the time (and the lessons are here) i recommend you learn how to build them yourself. But don't forget this possibility because you might want to build gov or edu backlinks which are a bit trickier and if you find a great deal then why not.
Most websites that are concerned with generated organic traffic from search engines will in some form another bought or paid for links. Either by employing an SEO company who buys links or sponsoring events in order to have a sponsorship link. The trick of course is to find a balance between paying for links and developing your own links. If you are thinking about buying high value links, make sure you do a lot of research on the site before accepting offers, high google toolbar pagerank does not necessary mean good value. Monthly rentals are the best because if Google does decide to penalise the site selling the links you can always pull out after the month. textlinkads and textlinkbrokers are good, buysellads also offer banner ads with do follow and some are pretty cheap. but if you have a decent budget of 5K+ per month than try lowprofilelinks