I would highly recommend not buying links from anyone other than a website proprietor. Search engines are getting much better at determining which links are purchased, and in some case penalize both the advertiser and the site selling the paid links.
The best way to purchase links is to pick up the phone and make them an offer. Be very polite, compliment their website, and then ask if they would be willing to place a link on their site.
I personally think buying links this way is very inefficient unless you can get a link back from a related industry giant with enormous amounts of traffic and visibility. Otherwise your time and money is best spent building really good content, guest blogging, and growing your social network.