I sometimes feel people think there is a box full of "traffics" somewhere, and some santa clause selling that "traffic".
To get traffic you need to move a person from where they are to your website, and if you want to pay for that then it's through advertising. The main advertising networks you'd have access to with your budget (I'm assuming) are Facebook, Google ads, and Bing ads (like
@se900se said above).
For the "niche" part, you'll be targeting your target market, or limit the ads (if display advertising) to be only on the website of your target niche. You can use other networks but then you're diving in a pool of confusion and complexity.
People who will sell you traffic either buy it from the above networks (expensive), or sell you fake traffic (it has his uses). Pick your poison.