I passed through there last week, and didn't notice any SIM vendors. (I wasn't looking, but in the past they were hard to miss.) I just searched on Baidu, and the general (i.e. 80%) consensus on various message boards is that it's not possible to buy SIMs at PEK any more. However, I simply don't believe this because PEK is enormous, SIMs are a high demand commodity, and Chinese merchants are pretty adept at catering to demand.

@OP: I suggest that you simply ask around (e.g. information desks and convenience stores) when you're there; someone will point you in the right direction. Assuming you succeed, be sure to get a 3g (Unicom only if you're using an iPhone that was produced before Apple got on board with Mobile's 3g standard) SIM. I don't think you want 4g because the last time I checked (about a month ago), 4g data was only supported in the home region of your SIM.