First, this repost:
Since not very long, I tend to specialize my search (exact positions) in youth academy and essentially do 4 blocks of 3 months; always Netherlands and 2 other countries per quarter.
Results are pretty good and realistic, though at first I tend to go for 15 or 16 year olds (unless it's a goalkeeper). This means youth players won't advance until the end of season 2.
South America yields great attacking players, Belgium gives great defenders, Poland very good goalkeepers and other countries are there because those nationalities happen to be a lot at Ajax.
But in contrast to @CO88LER5 I tend to actually not flood my youth squad with players, always keep it at around 9-10 players per time. And I'm very strict - they're actual OVR already needs to be over 60, while their potential at least needs to be over 80. If they fall below any of those two standards, they're released from the youth squad. Potential top players only. Then, towards halfway of (usually the 2nd) season, new youth players start entering the senior team (as per normal at Ajax).