Local tack/feed shops and if you live in an area that has a regional magazine ie Central Horse News then look in there as most organisers would advertise in something like that. Don't know where you are to suggest anywhere else, so can only let you know where I would look.
We look on horse dates. I also belong to Anglian Distance Riders, so any clubs local to you would offer rides too. I also go to church rides, often hear of them by word of mouth or advertised in feed shop.