You can find the mini medals in the field by searching in the grass and bushes, looking under rocks, searching for them in treasure chests.
You can trade your mini medals with Minimoggie cat in the bottom right cave of Alvolea.
The prizes are magic water-3 medals, Yggdrasil leaf 4-medals, Kitten Claw-15 medals, Poison Needle-20 medals, Gold bar-30 medals, Life Bracer 50-medals, Liquid metal armor 100-medals.