There are lot of sources for me: real life, imagination, Youtube, Google, pretty much everything.
When i drive (or walk) around the town i look at the buildings and cars. If i spot a nice car i try to remember how it looks. When i come home... For a building i also look at it and remember how it looks.When i come home i try to remember what details it had and try to incorporate them in my building. I don't build buildings, i lack pieces (about 800 or more loose pieces).