Hello Peter,
we have a friend, killifish breeder, in Indonesia who is able to obtain wild caught bettas mostly. I think the fish come from some small scale collectors and fish like that are not traded in big numbers, there is quite a market with such fish between the enthusiasts. His friends are sometimes able to bring such fish to Europe. I had no space to taky any fish from him this time, so I did not even check what was available. But I think two weeks ago or so, my friend told me that he has a group of parvulus, that he took from him and he wanted to give me a pair. Of course I had to make space for such fish, so I agreed, but before we were able to arrange the handover, he lost most of the parvulus and I got what he had left, two females and a male. So since that time, the fish recovered nicely under heavy feeding and with heavy water changes.
What is important for your question is that the Indonesian friend told me they currently know about three locations where parvulus live. The fish that I have come from Bukit Batu near Palangkaraya , Kalimantan Tengah.