If you can afford to pay a little money for the game, then the Subway Surfers Store provides you access to buy the keys using real money. Various forums and marketplaces also offer a discount on Subway Surfers keys, and you can use them to buy them. Else, the best way to get Subway Surfers keys for free is by using the MOD APK file provided here. This will let you enjoy unlimited coins and keys that too without spending a dime.