Maybe around Sheung Shui Station there might still be people selling those two-number prepaid cards, I don't know for certain. SSP you can look around, too. But on the whole they seem to have disappeared.
I've been through this and getting a Mainland SIM is the simplest approach. Note also that the Mainland number on those two-number cards often doesn't work for SMS codes and the like for technical reasons. Been there, too.