I've been getting treasure chests lately and I don't know where to find the keys to open them. Can anyone tell me where they are? My bank is about to overflow with these

There are 3 types of keys:

-Silver key (opens up 1 - 35 chests)
-Gold key (opens up 36 - 50 chests)
-Platnium key (opens up 50 -55 chests)

The keys are not obtained through drops, and you can only get them through
-The cash shop
-The web depot (when wasabii decides to send them out to every player)
-The purple egg through the Time token shop (I believe you can only get silver keys through this)