You can put your book for free on multiple vendors (Barnes & Noble, iBooks, Google Play) and try to get Amazon to price match, or use KDP Select if you want your book exclusively on Amazon.

Another alternative, though, is to give out Advance Review Copies to your supporters. These reviews are weighted less heavily than verified purchased reviews, but they'll still show up on Amazon.

Most people have success rates of about 10-30% of people who receive an ARC actually leaving a review, but this is hugely dependent on genre and your relationship with your review team.