So this website has a specific forum for it here.
https://www.backyardchickens.com/forums/animals-in-need-of-free-re-homing.16182/
I am guessing that this will work better if you are in a populated area, because it will be more likely that other BYC members will also live there.
I have used craigslist with pretty great results. That seems to work better in rural areas since there are usually more folks looking for roosters, but not always.
There is something called Front Porch Forum in Vermont, which is basically a listserve for each town to post announcements. I have actually had people pay me for roosters on there, just $5-10, but still something.
I am guessing by the light of probability that you do not live in Vermont since we have the second lowest population in the US (assuming you are in the US), but there are some other neighborhood forums I have heard of (like nextdoor.com). Maybe a web search for "neighborhood forums in [wherever you live]" may yield some results.
Good luck!