Belleville washers or disc springs are convex discs supported at the outer periphery by one force and an opposing force on the centre of the disk. This unique characteristic of the Belleville washers allows them to be used in multiple configurations to support the structure being built with the desired load.

Coil springs create energy when the coils are pushed together. The elastic material allows it to return to original form once the load has been lifted. These are used in common applications such as mattresses, toys, and shock absorber systems.

The difference:
Both have the same function but the main difference is their load versus deflection characteristics.

Disc spring designed to withstand very high loads with a small amount of deflection on the other hand coil springs can only accommodate lighter loads and much higher deflection.

Here is the supplier link where we usually get them https://itafasteners.com/products-washers-belleville.php