Order some tiny hooks online and go fish around weeds and near rocks where baby sunnies like to hide. Microfishing hooks work well for tiny sunnies. They'll bite on most things, but anything white and smelly will work especially well- bacon fat works if you can keep it on the hook. I use a tiny scrap of a Gulp crab bait, but any white or near-white lure will also work, you just need a very small piece.
Two days ago, I went down to a river near me and found a spot right at the base of a bridge that had a bunch of baby sunfish clustered around it. Took me about half an hour to catch 6 baby sunnies. The real difficulty, once you find a good spot for them, is IDing the tinies you've caught to be sure that you don't have the wrong species.
Alternately, if sunfish are considered nongame fish in your state (they probably are), you can use a dipnet or similar to dig around in underwater weeds and bushes until you find some.
If you get tiny babies all about the same size and put them into a tank with plenty of good hiding places, they'll adjust pretty quickly to aquarium life. Feed frozen foods at first to get them eating, especially bloodworms, and then try to wean them onto dry foods.