There are many website who provide Appstore/Google Play reviews. The problem is how to choose the best sellers,you should keep in mind these tips when buying reviews.
Real Users
You can not buy any real users reviews because the sellers will lie to you. There is no provider who sell real user reviews all making reviews through devices. You can not find a way to distinguish its real or not.
4/5-star ratings
You must choose 4 or 5 stars, some 1-3 stars seems more natural for your apps, but 1-3 stars reviews will hurt your app and make app rank down. Do not buy 1-3 stars reviews.Also, 80% of users would like to regard app ratings as an affecting factor when they want to install an app.
Prices
Choose the lowest price as much as you can , all sellers provde the same work and they just sell with different price, you can not get extral work with high price and which always spend much money.Of course, you should not focus on the prices only; you should also pay attention to their quality. Consider both factors entirely and make a smart decision for your app.
After-sale services
Reviews or ratings has the risk to be removed by Appstore and Google play, the clearify after-safe service seems important. Try to find someone who give promise with re-add plan. Before you buy reviews, it is better if the sellers can provide some free trial that is the best way to try as a start.
I strongly recommend try Appleaso.com which provide 10 ios reviews free trial. They also provide keywords installs can 100% improve the rank with keywords search.