We have experimented with catching our own minnows a couple of times. After you do that, you don't mind paying for them at the bait store.
We got a night light, and first time out with it, we were just amazed at how the minnows swarmed around it. It was mesmerizing to watch them, so naturally we figured all we had to do was get a dip net, a cast net, or a trap, and we'd have all the free bait we wanted.
It never really panned out like that. We've caught some bait, but those minners know how to stay out of trouble. Even with a large diameter, long handled dip net, you can only catch a lucky few. The cast net was just too much trouble to throw from the boat: you get water everywhere, it's hard to cast it next to the boat, and the light/cord always seems to be in the way. It's easy to drop a trap over the side, wait a while, and see what comes up, but it's just not a dependable way to get bait.
But don't let me talk you out of it! On those nights when the minnows are swarming around the light, it is totally cool to just stare into the water and watch them do their thing. We still have lines out, but for us, the entertainment is the minnows; it's kinda like watching the camp fire. Every now and then, a crappie or bass will flash through the light, and it will look like lightning. When you finally turn off the green glow and head back to the ramp, you'll feel like the aliens just released you from the mother ship.
Other people might have more successful stories. I hope some guys will weigh in with some good tips!