In Reallusion Content Marketplace or Google 3D Warehouse, there are a lot of content such as props, background models. You may export these models into UDK via 3DXchange5.
Go to "File" >> "Export to other 3D format" >> "FBX". Select preset as "Game Engine - Unreal" in FBX export window.
Set "Package", "Grouping" and "Name". Select Import Type as "Static Mesh".
Drag Static Mesh from content browser into 3D scene.