Go to for ambients are Poison Props, Midnight Syndicate, Peter Gundry, Cryo Chamber, Lucas King. I just added TabletopAudio to my list.
As my event is a party, guests spend a lot of time in each room talking with others. I mix a lot of music together to get the sound I'm looking for to be creepy and entertaining/joyful. Each room has a about 90 minutes of looping audio. Each track has ambient associated with that room (swamp, crypt, old house creaking, etc) and some rooms have music as well. My swamp area has banjo type music - a lot of Steve 'n' Segulls - and other clawhammer type banjo tunes. My vampire room has a lot of dark classical music mixed with classical takes on metal music (e.g Harp Twins, 2Cellos). All the rooms fight for sound as there is a dance room that pumps out dance music - all night. If I have time, I'll mix a lot of horror movie quotes into the music.