Looking at this article you'll discover the good and bad places to purchase links.
Let us get the bad stuff taken care of first. Link brokerage is generally bad. There are numerous reasons. First of all Google can easily detect those back-links using reverse engineering and penalize your rankings. Second of all, they often times link out to spammers and don't produce great results when it comes to rankings.
Now let's examine the great places to purchase links.
1) Paid directories: Spammers prefer quantity over quality. Payment often serves as an effective barrier to entry. This means that a spammer will try to put links on 1000 ineffective directories and avoid paying for the high-quality directories. Virtually all paid directories have staff that reviews submissions. Search engines like google like sites reviewed by humans and give them more trust. Most costly directories are dir.Yahoo.com and business.com.
Listed here is a set of secondary directories that it is worth submitting to:
You need to review them manually, because unfortunately a number of them change for that worst. During the time of this writing they may be still good. Also Look at the local directories (especially effective for local brick-and-mortar businesses)
This is what you should check: Do they accept all listings automatically as soon as you pay them? If that's the case it is not an excellent sign because spammers may buy links also. The amount of backlinks will the directory have? How many times does it get crawled by Google (check info.directorydomain.com to see last cached date)? What is their Alexa rankings (simply how much traffic do they get)?
It is possible to exchange links with directories, as long as they're vertical directories highly relevant to your industry. A nice approach is to produce a directory yourself and populate it with listings. Utilize it to hyperlink to directories which need a link back exchange for any free listing. Never link back to inferior directories.
Carefully examine the "directory submission services", in many cases they own all the directories that they're submitting. This will create poor backlinks for often astronomical prices.
2) Donations: Many non-profit organizations have a set of people who developed a donation. It is possible to create a small donation and get a web link to your website. A number of these places have common keywords on-site including "nfp", "npo" etc. Try looking for these keywords to find these organizations.