In Beijing, real UGG boots can be purchased at their authorized store at the Solana shopping mall, which is by Chaoyang Park and about a mile east of the East Third Ring Road (Westin Chaoyang and Kempinski Hotel areas). They will not be cheap. It is the same story for all GENUINE branded goods here--do not expect bargains for the real thing, and when you find bargains, they won't be real.
Most anywhere else in Beijing, and especially markets, may have fake UGGs--do not believe anybody who tells you they are "real factory overruns." Even if there were real factory overruns around, you most likely wouldn't have access to them through the secret channels.
Whether genuine or fake, UGG's are fugly.