They are called 'engineering split collars' in the UK, available on-line from engineering suppliers. Mine are 35mm internal diameter, 55mm overall diameter and 15mm thick. Each one weighs 170g, so seven (my current maximum!) weigh about 1.2kg. I got these as, when I got them, they were a lot cheaper than custom made ball weights available from adult sex shop suppliers! I hope that you get some and try them - it's an awesome feeling having over a kilogram pulling down on your balls, especially when walking around!