You can comment for free on a number of blog posts that do not have the no follow link - see my last sig link!
You may also consider contacting the top sites in your niche and asking them if they sell links. This way you avoid all the fees that companies like text-link-ads perceive.
Buying links is not good according to Google there are chances of getting penalize. Try to do link building naturally .
Buying links will get your site "out there". Once you're out there where your target market can see you, you have the strength to negotiate link exchanges with quality sites and continue to grow from there.
According to Google PR algorithm if you have link from site A to site B and site B to site A then that vote (link) cancel each other so link exchange will not help in getting good PR or rank.