I am not the one you wanna hear from since I've never had them but thought I'd drop my $0.02.
They are not common but not very rare. Not too expensive. A 1' one would set you back $150-$250 in the USA. They are far more slender built than a TSN (and are from a different genus) and in the wild they are a very specialized feeder feeding on nekton. My understanding is that in captivity they take the usual foods large Pimelodidae take. Here are some data:
Firewood Catfish Sorubimichthys planiceps, Brazilian local name, sorubim + Greek, ichthys = fish.
Siluriformes > Pimelodidae
FishBase: max 150 cm SL.
FWR: max 145-200 cm; 16-50+ kg.
As you can see, they can be very long but weigh relatively little as they are thin.
Freshwat.; demersal
Tropical
South America: Amazon & Orinoco River basins.
Mainly nocturnal. Feeds primarily on nekton. Fisheries: minor com; aquar.: P.A.
From Wikipedia:
http://en.wikipedia.org/wiki/Nekton
Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water (usually oceans or lakes) able to move independently of water currents.
Nekton can be contrasted with plankton which refers to the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute predators.
As a guideline, nekton are larger and tend to swim largely at biologically high Reynolds numbers (>10^3 and up beyond 10^9), where inertial flows are the rule, and eddies (vortices) are easily shed. Plankton, on the other hand, are small and, if they swim at all, do so at biologically low Reynolds numbers (0.001 to 10), where the viscous behavior of water dominates, and reversible flows are the rule. Organisms such as jellyfish and others are considered plankton when they are very small and swim at low Reynolds numbers, and considered nekton as they grow large enough to swim at high Reynolds numbers. Many animals considered classic examples of nekton (e.g., Mola mola, squid, marlin) start out life as tiny members of the plankton and gradually transition to nekton as they grow.