Don´t believe that PageRank is the only, heighly weighted factor for a high ranking in Google´s Search Results. It´s not - it´s one of more than 100 factors, and it´s not even the factor with the highest relevance.
In fact, linkbuilding is one of the most important factors. But not only the number of links to your site is important, is also important which site links to your site. The higher the trust-level of a site/domain linking to your site/domain, the better the value for your site/domain
Try to buy links, in many cases this helps A LOT and improves your ranking!