Search engines - this is one of the best sources that provides relevant traffic to your website. Increase your positions on search engines for specific keyword phrases with the help of link building , article submissions, general directory submissions , social networks. Google Ads - this is an expensive type of advertising, but I don't recommend to refuse from it unless it becomes not profitable for your business. Social Networks and social bookmarking submissions - this is a new way to get plenty of dedicated leads. Facebook, Twitter and LinkedIn are good places to share free bonuses and reports, provide a discount and let people know about your special offers. Are there any BETTER traffic sources better than the 3 i've mentioned?
You left out buying an ad/mailing to a targeted email list. This is a very reliable source of leads and buyers.
Banners at targeted websites? Those cost alot less than google ads for us at least. However, its more risky because it doesn't guarantee traffic.
You could always post up that you want to buy an ad/solo mailing on IM forums like these. Another way is to contact product owners in the niche you are in directly. Chances are if they have a popular product, or an established blog, they will have a list worth mailing to.