Is the site indexed? How many keywords do you need hit? What duration (hits x days)? Any requirements for bounce, pageviews and time on site? Do you want just Google or Bing, AOL and Yahoo also? If other search engines then what ratio (75%, 15%, 5%, 5% is typical)What about technical statistics (browser, screen resolution, color depth, OS, plugins)? How many Countries and Cities?
Just a few questions that come to mind as I built a network for such accomplishments. It's for private use currently but I might be inclined to offer it as a service one day.
The problem with real visitors for hire is they don't look real, if that makes sense. They must stay on the site like 4-5 seconds and typically hit one page. In analytics it looks like avg time on site 4 seconds, bounce rate 80-90%. With bots I have much more control and if I have time I typically run 2.5 to 4 mins on site with bounce rate around 30%. While I could have bounce rate at 0% it doesn't look natural. 25-35% is natural.
Just some food for thought really. When ordering organic traffic you should be prepared to answer all those questions so you're not disappointed after the service has been delivered.