I guess that would depend on why you need 40,000 website visitors. Usually people don't care about 'visitors' per say, they care about the quality of the visitor. You could easily buy traffic that doesn't do anything for you (unless your trying to boost your stats too sell the site.. but that's dishonest).
I would start doing a little adwords using phrase and exact keywords (broad will get you more traffic... but convert less and probably cost more). Then do a little seo; optimize your site, do some external link building.
Maybe run a couple give-a-ways or competitions. Spend $100-$500 on a cool coffee maker to give away. Promote it through social networks and this forum .