Thai-delight is probably the best authority here. But I can help you out a bit, based on what I know and what I've heard... plus a little bit of reasoned speculation...
The Thai ladies seem to congregate in certain areas, even certain buildings. They also tend to price a little lower, so they tend to advertise more heavily on the less expensive sites (161sex, miss148, 1212u) than on sex141. For some reason, they also seem to call themselves "Mixed" rather than "Thai"... But if they advertise as "Mixed" and speak English, it's a decent bet they're Thai (or Malay or Indonesian).
Here are some specific suggestions:
1) Search for 124 Fuk Wing Street in Sham Shui Po on 161sex... There are five WGs listed at this address. I know for sure that some are Thai. I suspect they are probably all Thai.
2) There's another love nest of Thai WGs at 63 Pilkem Street in Jordan. miss148 lists six WGs at that address. Some, if not all, are Thai... I think there may also be some Thais at 57 Pilkem.
3) I have not been down to Sai Wan Ho or Shau Kai Wan, but i have heard reports of Thai women down there, and there are some ladies on sex141 that seem to fit the mold... "Mixed", speaking English, lower rates than typical for the area... Here are some good possibilities there:
Rose...
http://www.sex141.com/en/viewid.php?uid=31016
Lita...
http://www.sex141.com/en/viewid.php?uid=31014
Ka Yan...
http://www.sex141.com/en/viewid.php?uid=27702 (in Causeway Bay; I'm not so sure about her)
Good luck! Please report back, regardless of the outcome...
Have fun, CGP