Was there last month. C41 colour film (Kodak, Fuji, and Luckycolor) can be found easily. BWs, Ilford, Lucky, Shanghai, and Era- last three Chinese made, and quite good. The place to go is to the "Beijing Photo City" mall. Films of almost all types and makes are there, and at good prices. You can even find film developing equipment and BW chemistry.
And if you're looking for cameras, you'd find a lot of Leica, Hasselblad, and even Zeiss there. Some shops even sell only the classic film cameras. You can find very interesting Chinese cameras there too- many you wouldn't find elsewhere, even on eBay.
Beijing Photo City Mall is on Wukesong Street, Haidian District.
As for xRays in airports, your film's going to be safe. I had some ISO 400s with me and they never got nuked. Expect your bags and cameras to be X-rayed in places like the Forbidden City and Tiananmen Square. Otherwise, there's nothing else to worry about.