Corrupted Dungeons

I've recently reached lvl 75 and wanted to do the forgery chest but I have no idea where to find the keys. Can anyone tell me where I can at least find the first infested pit, decepit sewers and lost sanctuary keys?

 

Trade market is the best way

 

You can find key guardians in areas of their respective levels, decrepit sewers in the dark/kander forest, infested pit in the light forest, lost sanctuary around gylia lake, etc.., keep in mind that some of them tend not to spawn often so you'll have to be patient (speaking from experience)
You can also buy them on the market but due to their rarity some tend to be quite expensive (namely sand swept tomb)
Finally you can get them through raid rewards or forgery chests
TLDR : just buy dungeon bombs lol /s