As far as I know, you're gonna have a hard time looking for desired low-poly things. As for a whole .upk package, I stumbled on a website once, they give udk packages away for everyone in need for free... as far as I remember. Epic wouldn't like people selling packages anyway.
If you want to make assets yourself, I'd advise dividing the meshes into parts (eg : pillars, walls, roofs, floor) so you can reuse them for different things