One of these? https://www.kjc-gold-silver-bullion.com.au/CT/gold-coin-pendants/83 Note that I've been told that, due to changes in the diameter of coins, these May not fit more recent ones. Better to confirm.
Have a squiz at sites like bezel.com (or maybe bezels.com ?) Esslinger might be helpful too. Some of those companies have tables containing coin sizes etc. Maybe measure your coins dimensions and type "bezel to fit coin x mm diameter, y mm thick" Or just google "coin bezels" and go from there. Or "coin bezel for perth mint 1/4 ounce gold coin"