I find playing in Multiplayer Boss Rush sessions pretty handy if you want to get quick keys.
If you don’t have anything to spend on items, don’t have any item equipped, and just join any games you happen to find playing Boss Rush.
IMPORTANT NOTE: Try not to get much firepower (asides from Charity) if you’re doing this though, stronger players need them to go through bosses quicker, thus giving you more keys quicker.
Also, if you die, and you re-appear as a ghost, might wanna rejoin so you can actually get the keys, as ghosts cannot get any keys, at all.
Or another alternative in v118: do Pot Luck whenever it’s avaliable.
It’s sometimes available in the Events section of the mail, and you play with a pre-determined set of items - which doesn’t cost you anything for playing, meaning all the keys you get is yours, no losses are made via items used.
Note that you can only do a Pot Luck Level once, so try and finish it for the max amount of possible key gain, and don’t be afraid if you have to use the given items. You’re only doing this for the keys after all.
With all of this together, it might still take you quite a while overall to get to that point,
so good luck, and have fun!
2nd best option: Supernova missions.
More repetitive and predictable, yes, but you’re far less likely to die there (except from maybe the final boss, but by which point you should have quite a bit of key gain already).
There’s also an investment on 2 Always On items in order to find Supernova missions though (they’re available if you can enter suns). Buy it from Shady Dealers/Academy for cheaper, prioritizing Shady Dealers since they have a higher price reduction.