In Den 66 near the Pokemon nursery, a Ditto will spawn only when there is the special purple beam, which I believe has a 10% chance of happening with any Max Raid spawn. This method is used to get 4, 5, or 6 IV Dittos. If you just want any Ditto, the best place to get that is at Lake of Outrage, where it has a 10% overworld spawn rate in any weather as Averyplayspokemon stated. The reason you are getting Zigzagoon is because the raids you are doing are not the rare raids.