You can get magic stones from several different events. The first free way that comes to mind is from dragon trial, which unlocks at level 100 (but there are many ways of obtaining them before that). A magic stone increases the stats for one of your heroes in the backup formation, by a certain percentage. I.E a level 1 magic stone increases the stats by 10% and a level 2 magic stone increases the stats by 15% and so on. If you're far from reaching level 100 yet, your best bet would be to check out events as they come and see if you can get any magic stones from them. There is currently an event you can get magic stones from though, the Christmas Exchange shop.