Negamation is a nega version of the world Omnimation. Like how Omnimation houses each Omnitrix Guardian, Negamation is the home to the Nega Guardians.
Due to Negamation being a nega copy of the world Omnimation, the locations are exactly the same as Omnimation. How you can tell them apart is that they have something that is opposite to the original. Like Omni and Tonto's warehouse is old and has cobwebs (that they refuse to clean), while Nega and Otnot's hideout is brand new, and doesn't have a speck of dust. But, the main differances between this world and Omnimation is that it has no links to other dimmensions, and that evil is the way of this world. In fact, due to the assorted amount of evil in this world, Heartless and Nobodies act like this place is home. Like say, you lived there, you were walking home from the store, and there a Heartless on the other side of the street, you could wave to it and say, "Good morning to ya," and it would wave right back.
But, it doesn't matter how much darkness rules this world, there always is a bit of light. Some people have formed a rebelion to fight evil on this world, but it's still kinda hard when the whole population is evil, but they still try.
Health Food Store
Nega Guardians' Head Quarters
Dark Mysterious Woods
Since this world is basically a nega version of Omni mation, it has the same exact Heartless, in the exact same locations. There is just more of them.