After reading this article you'll discover the good and bad places to buy links.
Let's get the bad stuff out of the way first. Link brokerage is generally bad. There are several reasons. First of all Google can easily detect those back links using reverse engineering and penalize your rankings. Second of all, they often link out to spammers and do not produce good results in terms of rankings.
Now let's examine the good places to buy 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. Almost all paid directories have staff that reviews submissions. Search engines like sites reviewed by humans and give them more trust. Most expensive directories are dir.Yahoo.com and business.com.
Here is a list of secondary directories that it's worth submitting to:
You should review them manually, because unfortunately some of them change for the worst. At the time of this writing they are still good. Also Check your local directories (especially effective for local brick-and-mortar businesses)
This is what you should check: Do they accept all listings automatically once you pay them? If so it's not a good sign because spammers may buy links also. How many backlinks does the directory have? How often does it get crawled by Google (check info.directorydomain.com to see last cached date)? What is their Alexa rankings (how much traffic do they get)?
You can exchange links with directories, but only if they're vertical directories relevant to your industry. A good approach is to create a directory yourself and populate it with listings. Use it to link to directories which require a link back in exchange for a free listing. Never link back to low quality directories.
Carefully examine the "directory submission services", often times they own all the directories to which they're submitting. This will create low quality buy backlinks for often astronomical prices.
2) Donations: Many non-profit organizations have a listing of people who made a donation. You can often make a small donation and get a link back to your site. Many of these places have common keywords on site such as "nfp", "npo" etc. Try searching for these keywords to find these organizations.
You'll be doing something good while getting links in the process.