Mostly i find good Altcoins at Bitcointalk forum. I think it is the best, because it's for cryptocurrencies and they should be annouced first here, then in other social media channels. Sure you can search in Biz and Reddit.
The second one is through the Youtube, because every ICO should have a company vision Video, and they are all linked to Youtube. For me these are the sources. But i will very like to hear from someone else, where they find the best ICO's.