If you want free fish, you could check out a local aquarium group/club if you have one. Some people specifically seek to rehome their fish as opposed to sell them. However, if you don't have a local aquarium group, your best bet will be FB marketplace. People will sometimes list entire tanks that they no longer want, fish included. Word of caution though: basically all of the free tanks/fish that I've personally seen listed on FB marketplace are at least somewhat neglected, so you could be taking on a rescue case and if you're not prepared for quarantine, medical care, frequent testing/water changes, and possible death of the fish regardless of treatment, then you'd probably be better off buying fish at a reputable LFS