In HK, you will find "walk up" girls and "hotel" girls.
"walk up" girls tend to be older (often locals), cheaper (500HKD), and located in grungy apartment buildings. If you try the walkups, you will likely end up walking up and down 10 flights of stairs, knocking on 20+ doors, before finding something you like. Also, be prepared to cross paths with 20+ other mongers wandering through the building, all knocking on doors. I personally do not recommend the Fuji building in Causeway Bay. The level of service Fuji is quite low (cbj). Other walk ups, like Hing Loong building in Central, are better.
"hotel" girls tend to be younger (often girls from China on 7 day visas), more expensive (1000HKD), and located in 3 or 4 star hotels in Kowloon (usually in TST, you won't find much on HK side). My recommendation is to stick with the "hotel" girls. Standard service is 45 minutes, bbbj (cim is ymmv), no rim.
You can find "hotel" girls using the sex141 website (where they are called "honey girls").
Here is the link to the current honey girls. You will notice that many honey girls have names that start with the same code (such as "XL", "DG", "FF" or "TR"). These girls all belong to the same agent (you can click on a girl's profile to see her "sisters"). ONLY deal with girls with coded names. And, instead of calling/texting the phone number listed, I suggest you look up the number on wechat to find the agent's account. I recommend the "TR" agent -- reliable booker with reliable thai girls.