Rutilans, as Bartian said: get a car, get a big plastic sieve with small holes so that no Paro could slip through and drive (to tell a secure region) to the east coast between Kuantan and Cherating. There you have to look for small ditches and running waters with tea coloured water (coming from former rainforest regions on a peaty ground). Take the sieve and try to catch in the riparian vegetaion (not in the free water), some in 30-50 cm, some only in one meter depth. Not at the surface as the other labyrinths. It’s not as easy as catching barbs or Trichopsis which you get in the free water swimming. In this region P. nagyi is still abundantly to be found, especially now after the spawning period. You find nagyi over there even in different forms; the Kuantan form differs from the Cherating form. The form of Pekan Nenasi is slightly different than the form from Cukai.
If I lived in Singapore, I should go there and catch my fish myself. Every year more small waters are polluted or destroyed. You do no harm to the populations if you catch some indivuduals and try to breed them in your aquarium.
There are other places for the other Malaysian species, but I should begin with nagyi; you cannot miss them, I presume.