*Note that these are 4G recommendations:
You can buy this from the airport, CSL shops, 7-11s, and major convenience stores.
More info of the prepaid SIM card: https://www.hkcsl.com/en/Discover-HK-Tourist-Prepaid-SIM/, https://www.hkcsl.com/en/csl-Local-Prepaid-SIM-Card/
You can buy this from the airport, China Mobile shops, and major convenience stores (7-Eleven, Circle-K and Vango).
You can get this at all SmarTone stores and over 900 authorised resellers over the city.
More info: http://www.smartone.com/en/mobile_and_price_plans/prepaid/charges.jsp#data