Try one of these. These are the ones I use.
Tapjoy (huge install base, can be bit costly)
Featurepoints (larger volumes)
waypedia (cheapest, low volume)
You have a ton of info on this subject along with other companies on mobyaffiliates.
Installs here are all incentivized. Meaning; these companies pay people to install apps on their phone and you pay the company directly to connect you with these guys and do fraud checks.
Consider submitting your app to app review sites. There's free lists (google it) and do it by hand, hire freelancer or use a company like app launch.
While you're not doing PPI app review sites can still bring a ton of fixed-price traffic. That is if your app goes vita on any of them. They're split in two. Free and paid submissions where free isn't guaranteed and doesn't bring much volume. Paid submissions range from 50-200$ and guarantee a review, front page post for few days with optional features as Facebook, twitter, email and youtube advertising depending on the site.
Don't buy fiverr reviews,ratings,shares or anything else impacting your rank. That shit is all bluestacks (virtual machines for phone) and will get you in trouble fast.
Good luck.