Thanks to Joe Konrath’s latest post Tend Your Garden, I did some homework. I’ve put together links to the book boosting companies that people in the comments have used and recommended. I not having released my first book yet, can not comment on them from personal experience.
These first three seem to allow books with few or no reviews yet.
These two seem to require reviews, or at least it helps in their decision of whether or not to pick your book up.
Also something to consider if you don’t sign up with KDP Select, which means you can only sell on Amazon for three months at a time, is Flipkart. This is the Amazon of India.
If you have found any other useful ways to promote your book, leave them in the comments.