I do them at Jigid.com You can do them without joining, but if you do join (free) you can save them half-done, add your own pictures, and do other things.
I have downloaded apps on to my ipad. Just search for 'jigsaw puzzles' and you get a selection. Usually they are free with a limited selection of puzzles and after that you can buy more for a small amount, say 99p for twenty or so.
My favourite app is Jigsaw Box.
Of course, you don't get large puzzles because of the limited size of the ipad screen. Maximum about 250 pieces but the difficulty level varies. I probably finish a difficult puzzle in three or four hours.
I don't think you need an ipad (although there are jigsaw apps available). I used to do online jigsaws with my grandson on an ordinary computer using a mouse. Just google "online jigsaws".