No one can stop you from buying reviews, but it’s against App Store rules. Your app may be expelled from the App Store (according to the paragraph 3.10 of App Store Review Guidelines).
If you decide to manipulate reviews, you will be doing it at your own risk. Here you can check out app review service providers and their prices.
We don’t recommend buying reviews for a sophisticated product or an app in a foreign language - comments might be irrelevant. Some providers may post reviews prepared in advance. Confirm all the details with your provider.