- Roll20 message boards for online games
- meetup (I think it’s called) is a website for groups of people sharing similar interests and has some DnD groups on there especially in more populous areas.
- I believe both Reddit and Facebook have lfg dnd pages
- uk role players used to be good, not 100% sure if it still is
- a lot of local hobby shops (forbidden planet and travelling man in my area) normally have notice boards and you generally see dnd notes up there or can post your own
Probably a few others as well but the above will give you a good start.