I'm still new to the follow/unfollow strategy but for now I chose to follow the followers of large accounts from my niche (usually 15k+ followers) that also follow a lot of people so I know they also raised their accounts using the same method. This way I know I'm following people that tend to follow back. Tried it with board followers and those that pinned popular pins for a short while but the follow back rate was horrible (I'll probably try again later when my account grows more). Either of these follow sources may not get you the best quality followers but hey......there is strength in numbers.
Getting real niche-targeted followers is very difficult. For that you'd have to pick accounts that pin content similar to yours and have a high number of followers (50k+) but follow only a few themselves. This means the followers they got are real, interested users not follow-backs. Sadly, this also means very few of those people will follow you back and your account will grow slowly. Hope this helps