They aren't much traded. They're more or less grey in colour so aren't as popular with the kinds of people who keep arowanas. They're also more difficult to feed; in the wild they feed primarily on zooplankton (like daphnia) and to a lesser extent on tiny invertebrates sifted from the mud (like worms). While they *may* adapt to pellets in captivity, it can be challenging weaning juveniles onto such replacements, and given their size and appetite, they need a lot of food to do well. In this sense they are a LOT like pink gouramis -- it isn't a problem so much a problem finding the right foods for them, it's more difficult finding *enough* food for them in the long term.
In any event, here in England I have seen them sold very occasionally at stores like Wildwoods. They're relatively inexpensive, but as with any "tankbuster", the cost of the fish is trivial compared to its upkeep, which will be very expensive to do properly.
Cheers, Neale