How look accounts for advertising?

I want buy advertis, but I can`t find accounts with audience from a certain country

Look for trends/places/groups/celebs which are popular in that country, check out the pages related to them and send them a DM.
As a page dedicated to a particular place/person would probably be popular within the same region.
That should help, if that still doesn't work, let me know and I will help you in finding some pages.