I don't know where Dreepy is since that was a happy accident, but here is an image showing where you can find the Ditto and the Gigantimax raids:
You can see the Ditto raid spot is to the right of the path that goes down to the daycare center, as you head away from the daycare. It is kind of tucked into a corner.
The raids have a much better chance of giving you better IVs. I think how it works is that if it is a 3 star raid, it is guaranteed at least 2 Perfect IVs, 4 star guarantees 3, 5 star guarantees 4. There is a technique to resetting the Ditto raid spot until you get the Ditto. Ditto is the only one that shows up in the darker red beam. So you first save the game. Then you put the wishing star piece or whatever in the den, and when it asks you if you want to save, say yes but right away hit the home button so that you go to the Switch home screen. As it backs out of the game, you'll see whether it is a special beam or not. If it isn't a special beam, close the game and re-enter and if you did it properly you'll be able to try again. If it is a special beam, just go back into the game and do the raid. I've gotten 3 4IV Ditto doing this so far, and I haven't really spent that much time on it.