Depending on what you have accessed and which farm quests you've done (if any) the places that have machine exp are: Sunken Tunnel, Thriller Ruins, Access Glacier, Proxy Island, Chaos Brain, and Transfield (area 4 and Chaos area).
Areas listed above go from least amount of machine exp to greatest amount. KnightChessmon B/W (Thriller Ruins), MetalTyrannomon and Mametyramon (Access Glacier), and Solarmon (Proxy Island) all show up after various farm/species quests.
Digimon World Dawn FC: 3952-4851-4028 Why do people answer things they know nothing about?
about solarmon in Proxy Island, he is not at all a good source as he rarely appears. when i was hunting him for my Alphamon, he only appeared maybe every 5 battles or so. Just lettin you know, if your not at CHaos Brain or Transfield, the best place for Constant Machine Exp is Deep in Sunken Temple, but it really all depends on how far you are in the game, and what digimon you are able to handle