These Neodymium Disc Magnets known as Rare Earth Magnets have a strong 0.50kg pull.  Don't let the small size fool you.  Measuring 4mm dia x 3mm.  Perfect for those nano caches or even sightly larger ones.

Sold individually

The Ultimate Magnets! Neodymium Iron Boron (NdFeB) is used to manufacture the most powerful magnets per unit volume that are commercially available today. It is used where a small size and maximum power are needed. Neodymium Magnets exert very high forces and can attract each other through unbelievably large distances making them very popular for a wide range of applications including. They also have an immense and unbeatable resistance to being demagnetised, making them ideal for repulsion applications as well as attractive applications.
The magnets are triple coated (NiCuNi) for maximum protection against corrosion.