Hi there,
Two things come to mind: 1) Go to the networks directly and ask them about their fill rates, historically, in your target countries. 2) Once you've identified 3 that seem pretty good, ask your programmer to implement an ad cascade. S/he can probably handle this with a .plist file if you don't want to build a backend. Basically, you designate your preferred ad network (based on fill rates and eCPMs though the relationship between the two is a whole other can of worms), and then your #2 choice, and #3 choice. The "cascade" effect comes into play if your #1 choice doesn't have an ad to serve, and the code then queries #2. And so on. Rather than pick one ad network and hope for the best, you can have three or more networks that provide "failovers" and minimize missed impressions. Some devs also like to create an online dashboard so that they can swap positions in their cascades on the fly.

Hope this helps,
Austin

I would take a look at inmobi