I don't know if this is shared, it's pretty simple and I feel stupid I didn't do this before. For a long time I've been selling high pr backlinks and recently i got into relevant backlinks also, and there is a simple method I do to find them. I just search on google for: "my niche" blogspot (for example "android widgets blogspot") and that's it. You should get a lot of blogs with comments enabled and you can leave your relevant backlink in comments over there manually. Hope this helps.