You can buy SIMs in Sheung Shui (as suggested upthread) but rather than a random street vendor just go to a 7-Eleven or even a phone store. However I presume you can last an hour without a phone connection while you travel to wherever you are staying. So if roaming isn't what you want, then make any essential calls before leaving the mainland and get a SIM when you arrive at your accommodation.


However if you really only need to make one or two calls then just call from your hotel. Local calls between landlines are free and lots of hotels have phones in the lobbies, or at least they used to, and they shouldn't charge for calls from the front desk usually.