I need to know where the best place to get joy pendants is
and oh yeah I already beat the forsaken fortress so there aren't any enemys there

Just go anyplace indoors where there are Bokoblins, such as a submarine, or the first room of Dragon Roost Cavern. Use your grappling hook to steal the pendants from them, leave the room, repeat.

Pretty much what PooOnShu said.
Though I think there are several side quests you can do with them as rewards, though I can't remember any specifically.

Well once you get the hook you can use it on certain enemies for it. But lots of treasure chests just have joy pendants in them too. If you work to get them all early on then such treasures afterwards just become a let down. (so use the hook, and keep playing-Joy will come to you)

grappling hook + boblikin = joy pendant.