Blog Comments is free. You can do it yourself if you have time. Google doesn’t like paid links,so purchasing links from sites is not a good idea. Buying links from sites directly is corruption in terms of google. Commenting on blogs will give you natural backlinks which is taken with high value by search engines. I would suggest not to comment on blogs who automatically approve your comments. This will give you a low quality backlinks from spammed pages. There are 100s of software people use to spam blog pages and if you place a comment on such pages it is jut waste of your time. Well, blog comments in relevant pages with anchor text is best idea to boost your SERP.
1. No Spam Comments. Relevant Comment shouldl be posted after reading the blog post.
2. Avoid Viagra or Pharmacy Links.
3. Try to find pages with Low External Links. Less than 30 is the best.
4. Do Follow Links. Do-Follow links are always better than no-follow, but nofollow comments still count much (Depends on site authority).
5. Follow up your comment, reply them. Interaction with the bloggers is the best way to get traffic + backlinks and boost your serp