Decryptors are an alternative to Keys as a way to unlock Crates in Rocket League and they can be used for any Crate in the game (including Event Crates). The only way to get Decryptors is to buy them with the currency that you can earn during events. You can read more about events here.
But remember: any Crate Item unlocked through the use of Decryptors will be untradeable. If you want to get a trade unlocked item, make sure you use a normal Key. Also, if you trade in 5 items for a new item and at least 1 of them is from a crate you opened with a Decryptor, the new item will be trade locked.