In my case Im buying App Installs, Reviews, ratings for android & ios applications from a company called 9xen if you will search it on google i hope will get it their official sky pe : app.ranker
Im Hiring them since last 5 years and getting good results, because they send all humun traffic through mobile applications
till date i get around 1000 plus apps advertising so all time get good service,support and these all
their features of service like self manage portal,add fund, create self campaign, pay for each install ( CPI), track like each install,reviews,ratings etc
Im not sure about their rate what they will charge from you but in my case its comparative good, may be they given this on bulk volume
my current rate charge by them is
FOR android
1. Installs - 0.05$
2. Ratings and reviews - 0.10$
FOR IOS APPLICATIONS
1.Installs - 0.07$
2. reviews & raratings - 0.12$
May be this can help you