It took me some time, but I got two players (5/7*) at least for every single position on the pitch. The most difficult ones to find were the MR/AMR and ML/AML. The rest was quite easy, including the AMC/AMR, AMC/AML. Average age is 25, so I'll be replacing couple players next season, but that has never been an issue.

If you really need a specific position, the idea is to check back every 1, 2 hours (even with your mobile device), and you'll eventually get it. Don't just simply check at the exact same time of the day. That tend to take very, very long time if you're unlucky.