Hi Emma,
Casing parts are harder to find, and generally quite expensive for just a cosmetic touch-up. I know rounders has quite an extensive parts collection, including all the colors the mini originally came in. To prevent from ordering the wrong casing, drop them an e-mail with your exact model, and they’ll be able to help you out.
https://rounded.com/sony-ericsson-xperia-mini-pro-spare-parts/?sl=en