Hey there!
I'm Andrea, an indie app developer who just enrolled in a cross promotion community called Tappx. Since it's based on cross promotion, the bigger the community becomes the better it will be for us, its users. For now, it's been working really well for me and the popularity of my apps has slowly increased. For those who don't know how cross promotion works, it's very simple: I advertise your app in my app, and you do so in yours.
Tappx gives you credits for advertising other apps in your own app, and you can choose where you want to spend your credits (Tappix). You can choose the country you want to be advertised in and design your own promotion campaign.
Furthermore, if you don't have any banners of your own, they have some available ones that you can freely use. It won’t take more than 20 minutes to upload your app there and start making some profit out of it.