I'm leaving for the Trans-Siberian Railway in two weeks!

Apart from endless fantastic photo opportunities, it has been pointed out to me that this is a good chance to pick up some cheap+interesting gear (brass lenses, maybe an Iskra) that wouldn't normally make it to the west or wouldn't be worth shipping. I have 3 days in Prague, 6 in St Petersburg, 2 in Moscow & 3 in Beijing. Plus 4 in Mongolia and 5 in Guilin/Yangshuo but I don't expect so much shopping there Does anyone have suggestions for interesting secondhand gear shops in these cities?

Of course if there's something that I must go see/photograph in any of those places that you think I perhaps don't know of, by all means I'd like to hear about that too.