The keys spawn in hard to find places such as corners or behind things on the street where the door is or a nearby street. If you want spoilers, you can look up the key spawns on the Glitch Strategy Wiki. Happy Glitching!
www.glitch-strategy.com/wik...
This gives you where all the keys fits in to where, and where they spawn.
You can also buy them from other players via their towers or on the Auction.