These are the rattles that I use in my Animal Rattles
and in my DIY Duck Rattle Kits
These rattle inserts are 38 mm in diameter (1.5 inches) and are 11.11 mm thick (7/16 inches). For smaller toys I recommend the 19 mm diameter (3/4 inches) inserts. Those are 12.7 mm (1/2 inches) thick. You can see the size comparison in the last photo. Find them right here: https://www.etsy.com/listing/151462594/3-pack-of-small-rattle-inserts
Safety Disclaimer: I've sourced this component from a toy making supplier and it meets industry standards, but I can make no claim as to its overall safety.