Got a flea infestation, and neem & pennyroyal oils (with a bit of help from eucalyptus) always worked well in the West.

But I don't know where to buy in BKK.

Can anyone help?

look for poisian (the stuff thai do sniff) - it has eucalyptus, mentol, camfor, peppermint and the other oils. In shops from 6 baht

the best place to buy neem is down in Georgetown, Penang. Perhaps you could try the Indian markets of Bkk though, I bet they'll have it