I arrived at PEK Terminal 2 on Air France about a month ago, and I saw two stalls in the baggage claim area selling SIM cards. I don't remember the exact pricing as I decided to go to a China Unicom store in the city instead, but it was definitely lower than the one offered by klook from Daniil's link above (think about 200CNY for unlimited data and 100 ...
At the airport
From recent sources, there should be areas where you can buy a SIM card in terminal 3, none in terminal 2.
A TripAdvisor post from 2 years ago:
I believe that the only place to buy a SIM card at the Beijing airport is at terminal 3. It is in the area where you wait for your luggage. It is a China Mobile counter. If that does not work for ...
The prepaid data sim card wiki on fandom.com has them all:
Prepaid Data SIM Card Wiki
Including the ones you can buy in Hong Kong.
nb. it says "data" but for China it doesn't really make a difference.
You might want to get a Wechat account with wallet for easy topup btw.
On 8th Aug 2019, I bought a China Unicom prepaid data SIM card at Lo Wu Shenzhen Border. It cost me RMB200 for the SIM card while getting a usage credit of RMB100. The phone number was +86. I used a non-PRC passport to register. This package automatically deducts RMB6 for 800MB, which was the data usage incurred on that day. If there's no data usage incurred,...