Nothing like catching your own bait, or in this case your own fishl food. If you are interested in catching them, here is what you do:
1. Go to Wal-Mart buy a "Super Spreader BAIT net" (make sure it is for small bait and has smaller holes), usually made with green netting. Get a 6-8 foot one at least. Should cost under $20. Or you could go to your local bait and tackle, expect to pay at least double for the same net- but pick up some fishing tips while you are there. If your bait and tackle won't tell you where to find bait, go back to Wal-Mart.
2. Learn how to throw a cast net by practicing in your yard. Watch out for small dogs. Here is a video how, (the website is not completed):
The net will also come with instructions. (The 2nd video is the best).
3. At night, go to a bridge with a light that shines down on the water, and try there. Underwater rocks will destroy a net, so sometimes it is best to ask the advice of local fisherman, or see where they throw for bait. Bait fish, (especially greenies and silversides) tend to be attracted by light. If the net is not long enough, get extra nylon tie that is not too thin, or it will hurt your hand.
You can probably catch 1000 or so in an hour if you are in a decent spot. Very, very easy. If you are having bad luck, I have found that dock fisherman will gladly part with fresh bait they have netted for pennies on the dollar. After you get your fill on greenies and silversides, try running a sabiki rig near rocks, (watch out for snags), to catch smaller tropicals if you live in the right area.
While your local LFS will certainly say this is bad for your tank, consider where they got their fish...
Not coordinated enough to throw a net? Bait can also be caught easily with a sabiki rig. Buy the rig, (they have that too at walmart), try not to get it tangled when putting it on the line, and run it through the water, especially near lights that shine down. Harder than using a net, but easier than catching "keeper" fish.
Thought I would throw that out there.
All the Best,