You can also contact SEO companies which own networks of websites like designSEO.ca.
Not all SEO companies have networks of content which they can sell links from at a moments notice. There are quite a few small / amateur SEO companies out there just doing basic SEO via Twitter / Facebook, etc... which is really confusing Social Media Marketing with SEO and not realizing there is a huge fundamental difference.
There is also companies which sell Black Hat or a mixture of Black Hat/White Hat links, but these companies are just amateurs who don't really know they're doing more harm than good. See Anyone who uses Black Hat SEO is an Amateur.