In Macau, there are a few prepaid mobile/telephone sim card vending machine when you exit out of the Macau Ferry Terminal
In Hong Kong, you can buy SIM cards in Tsim Sha Tsui subway station on the Mass Transit Railway (MTR) in Hong Kong. It’s selling SIM cards for mobile phones, both the GSM and the CMDA variety (for those visiting China and wishing to have CDMA). Cards cost, mostly HK$100, which is about US$13.
Almost all Hong Kong phones are sold ‘unlocked’ meaning you can switch SIM cards at will.