use a software for blog checker and do avoid the spam. just check the niche of every blog you visit or simply you can check the page source.. its in HTML format.
Find blogs of Your niche in Google and also install Firefox addon like "search status" which has feature highlighting "No follow". When you get some blogs open them one by one and see their comments page. You have installed search status it will highlight comments as "No Follow" If this happens leave that blog and move further like same.
I suggest you to install the searchstatus tool of google. By using it, you can easily find the no follow links and do follow links.
It has become hard to find do-follow blogs/sits these days. Though the above suggestions will get you find blogs and sites with a definite attribute, you will notice that most of them are designed to no-follow attribute only! So instead of struggling to find do-follow blogs try searching for sites that has CommentLuv enabled. Sites that have CommentLuv enabled are much better than no-follow blogs! I can explain how to identify such websites but do you research will be more fruitful when you find them! All the best!