I'm no expert on intelligence-gathering, but here are a couple of ways I've been using to identify the highest-volume traffic sources for geos I'm wanting to scale for. Hopefully this will draw more tips from those of you that are more-experienced in this area!
1)Alexa Top Sites
http://www.alexa.com/topsites/countries
I'd just choose the geo and go over the top 50-100 sites by hand. There must be an easier way - perhaps by using a site like buildwith.com to identify the ad networks each site uses.
Anyway, I'd look through the sites for a)traffic sources themselves and b)sites with ads on them - and obviously find out which network the ads are from. Sometimes just by hovering your mouse over the ad you can figure out the ad network from the url.
Alternatively you can use a plugin like "Live HTTP Headers" (available for FF and Chrome) and click on the ad to see the redirects and figure out the ad network that way.
2)Similarweb Top Sites
The best thing about this tool is you can specify "ad network" as the category and see the top networks for a given geo. Not all geos are available but all the major ones are. For example here are the main ad networks for Argentina:
https://www.similarweb.com/country_c...com/ad_network
3)Quantcast (Contributed by TeamAragon)
https://www.quantcast.com/top-sites
Can filter for mobile-web browsers only
Other Resources:
Top million sites in Alexa (Contributed by bobliu):
CSV:
http://s3.amazonaws.com/alexa-static/top-1m.csv.zip
Web GUI
https://domainpunch.com/tlds/topm.php
Hope you guys find these useful! And if anyone could share similar tips, that would be much appreciated! Thanks in advance.
Amy
Seems like the SimilarWeb method is much simpler if you just want a general overview 
I personally find this very useful, thanks Amy! 
so much value. thanks a lot amy!
YES Alexa is a GREAT resource for everyone involved! You get to see site rank and which countries you get traffic from. You can use it on your own site as well as prospect sites!
Here's the top million sites for the hardcore guys out there:
CSV:
http://s3.amazonaws.com/alexa-static/top-1m.csv.zip
Web GUI
https://domainpunch.com/tlds/topm.php
It amazes me how alexa didn't bother changing the location after all this time. Thank you so much for the share!Did anyone do a comparison between Alexa and Similarweb results?
Similarweb seems great for "standard" websites such as news portals, company websites, e-commerce etc. and the numbers look pretty accurate.
But for affiliate marketed websites the numbers are nonsense for the few websites I've checked. 
I don't want to sound bad, I mean Similarweb is still amazing and even the free stats can be incredibly valuable.
Alexa was nice maybe 5 or 10 years ago but I don't think it's very relevant today, especially because Similarweb has much more sources of data.
I didn't know Quantcast before, I guess it can be good for U.S./Canada targeted websites. Worldwide there's a lot of websites missing. 