One of the best strategies to boost your keyword and app ranking is thru burst campaigns (buying downloads). Some of the providing companies use bots or software, which can turn out in your Developer account suspended for fraud. The best way to make this kind of campaigns is getting real users. How? with incentivized installs. Users get paid for downloading your app. Typically the traffic can come from the entire world, as these incentivized-install providers are owners of reward apps.
You can find very good prices, .05 cents each download is the best I could find. (So you can use it as a reference when checking provider companies) Traffic comes from not bad at all countries: US, Russia & Brazil.
Hope this info can be useful for someone! Good luck!