As far as I know, there is no such site that will search for you. There is, however, three programs that will search: Comment Kahuna, Fast Blog Finder, & DoFellow. They search the Yahoo/Google index and then scan the comments section of blogs for rel="nofollow" tags. There are also "dofollow directories" which only list sites that employ no-follow free commenting.
I dont thing there is any tool available for finding dofollow blogs. but you can search on google , there is some forums out there where users share dofollow blogs. You can search m sure you will find nice amount of dofollow blog lists.
Seo quake firefox plugin is a good option to find dofollow sites easily. It will also provide you some useful details (links, PR etc.) about sites you visit.
Dofollow blog finder doesn't show proper result some time they can also show nofollow bog. I use quirk search statues which can highlight no follow blog with read color.
Cant say much, it really is very hard to go on finding a list of blog providing do-follow baclinks along with getting those comment approved as well........
I think it is best tool for finding do follow blogs:-- http://mrdefinite.com/free-dofollow-blog-finder/ Have you tried it?