After many years of owning aquariums and some time spent working in pet shops, I can tell you that most of the fish you see in pet shops are imported. There are several species of freshwater fish being farmed but no saltwater fish that I'm aware of. A generous estimation would be 10% of the freshwater fish. With the exception of goldfish and Japanese koi.
Most of these fish are caught in other parts of the world. Alot of freshwater fish come from South America, the Amazon river to be precise. And most saltwater fish come from the far Pacific, some from Hawaii.
In both cases, the collection methods vary from chemical poisoning, mass netting, to kids on tropical islands netting them one at a time in tidal pools. None of which are gentle on the fish. That why there's such a high mortality rate. I saw a National Geographic program on this a while back and they stated that the guys catching these fish get as little as a few cents apiece for them and that as many a 90% die before ever reaching the marketplace. Many species of fish are endangered in their native areas because of over collecting for the aquarium trade.
There is more known about the nutritional and captive environmental needs of freshwater fish so you'll have more success keeping them healthy in an aquarium. Those same needs and requirements for saltwater fish, however, are less known. Feeding saltwater fish the proper complete nutrition for long life is still the hardest thing to do. With the proper equipment, you can get close to water and lighting conditions but feeding is another matter. Yes, some are easier to keep than others, but as a general rule more will die than will live in the long run.
Having said all that....If you plan to catch your own, check the local laws. It's is illegal to collect fish in some areas. I have a friend who collects his own fish off the jetties in Galveston. He uses a 12" net for the fast swimmers and a slurp gun for the other. A slurp gun is a sort of large vacuum device. Think of a large syring with a opening in the barrel. Pull the plunger and it sucks the fish in. Either method is very traumatic for the fish!
A fighting fish, eh? What about Bruno?