Paid traffic is important part of internet marketing specially if you use Affiliate networks. Some good networks to buy traffic. Not every network work for everything. Each work for different niches.
adwords.google.com
adcenter.microsoft.com
Looksmart.com
7Search.com
Facebook.com
pof.com (dating niche)
adMarketplace.com
Clicksor
Lead Impact
Infolinks
Kontera
Advertising.com
BuySellAds
Adengage
adpepper.com
Here's a chart I found which can be more useful. I didn't make it found it in Google image search.
http://img.techpowerup.org/130312/Traffic%20Source.jpg