I get these - just right for a syrian.
Amazon.co.uk
The one hamster I had who wouldn't use one was a one year old rescue (a bit insitutionalized! He'd been in the rescue a long time). He was very neat and chose a particular place at the far end of his house as a toilet. When I put the litter tray there he moved it again! Eventually I buried the litter tray under his pee corner so he couldn't see it (but I could still lift it out! And he dug it up and moved it again! Then he started using it and his old corner as well. Eventually he got the hang of it and used it lol.