Well I didn't want to add any maps to EA MonsterBytes that were not provided by DME, as I didn't want to include player created maps without their permission. But I would recommend visiting
EA Central as they have plenty of maps to use. Also note to the right of the forum there are a couple more "EA Resources" to look through.
And if you download any maps to your computer it is a simple as placing them in the "mapFiles" folder in EA MonsterBytes to have them show up in MonsterBytes.