How to get more recruitment medals?

If you mean the tickets that you use to recruit heroes then you will get a few of these free to use everyday and you can gain more through wither rewards or paying money. The arms supply paid subscription provides tickets on various days and there are other paid one off packages that give you tickets.

For free though you can get some everyday in the daily task (3 normal tickets at the 360 points level and 1 advanced ticket at the 500 points level. The daily supply box also has 1 normal ticket. The clash of zones event has an elite ticket available in the 3rd box. Killing the boss zombies (eg wendell the clown) will reward a normal ticket each day. There are also other events that come along from time that offer tickets for performing standard activities. Keep an eye out for these.