Most all my SR and quite a few UR units have come from the free silver keys they give out. I went threw a very slow lull in good stuff fron silver chests around the time they put the kibosh on the amount of silver keys you could get from the campaigns. I remember when you could drill the silver chests on campaign missions for hundreads of silver keys a week. This game goes in spurts, it's either feast or famine. Just last boss battle I picked up 3 SR units fron silver cheasts, this week today I have already picked up one SR unit from silver key.