Check your state laws first. In many places sunfish are considered gamefish, so if you take any home they must be taken by hook and line. In other places it is illegal to take live fish home at all.
That said, juvenile sunfish can be easily collected with a dipnet, seine, or funnel trap. You have to hit the right habitat, which for most of them is shallow margins of ponds and lakes, weedy swamps and backwaters, eddies of streams and rivers, etc. They tend not to venture into open water as much as adult sunfish (and juvenile bass) do.