Well I plan on buying it before heading to DPRK in Beijing. Unicom looks like the largest company. I think I do remember some place at the airport selling SIM cards at PEK, but I have only arrived through terminal 2 and not terminal 3 which I will arrive in April. My knowledge in Mandarin is extremly limited though so there is a reason why I would try at the airport where there is probably more likely someone would speak any English at all.