If you're going down the pellets route, try to get the pellets that suit hwere you catfish feeds. If your fish is used to feeding on the bottom, then try to find large sinking pellet. I'm unsure on brands and pricing avaliable to you, you'll have to look and ask around. They do, however, adapt to feeding off the surface, and floating pellets are loads cheaper. You have a pretty big choice between koi or carp pellets, as well as dorumin.
As your fish gets larger, you might want to look to feeding larger, meaty foods - things like chicken liver, pieces of (defrosted) frozen fish, squid etc. This does put more strain on your filter, so good water quality management is required.