A lot of people use PPC from big company, like google adwords,Yahoo! Search Marketing, and MSN
adCenter. They can provide your website with huge volume, but with high cost. You can find small PPC company for lower cost. But you must check where does the traffic come from. You must check the traffic quality. Does they do have click fraud or other bad way to get traffic. You will be spending money with no result.
I search the internet and found a PPC company 7search.com, which I think provide effective traffic. I tried using their PPC campaign, and quite impressed. You can see how much your competitor bid for a keyword.