I recommend starting with the smallest you can find-that's usually 2.5 or 5 lbs / 1.25 or 5.5kgs depending on your country (kg disks will be a bit heavier). The ones I've purchased in the USA and China have a rubber coating which helps with slip prevention when used as a balancing tool.
Bean Bags
I find these slip more and aren't heavy enough to really train the muscles around your neck, but it's easy to find a rice bag or fill a sock with beans to have a tool that will help with your awareness of head position. If you make a long tube and tie the ends together so it's shaped like a doughnut (creating a hole in the center), it will not slip off your head so easily. Using fabrics like velvet that will grip your hair can help too. If you want to increase your weight, you could try stacking some books on top of the bean bag.
Gatorade/Water Bottles
If you can find a water bottle with a wide base (the sports drink Gatorade bottles are pretty good for this) fill it up partially with water or sand or stones (water will slosh around and challenge your balance more, other materials can be heavier and help improve neck strength). It might slide a big on your head so you can rough up the bottom or rub something like candle wax on the bottom. This would probably also work with some tupperware containers that have a slightly cupped bottom.
