Lancer have some nice buildings in their catalogue, you might need to look at model railway stuff for urban style buildings though. The Italeri Berlin apartment block is useful however, if rather pricey.
If you make your own then you will need to find things like window frames and shutters to match, plus small balconies and those metal grilles that are so popular.
Sundaysoldier makes lovely scratch built stuff, it might be worth dropping him a private message, he has a thread on here with photos of some of his buildings and they look very good.