I think this is what you’re seeking; if the below link does not work, go on eBay and put this into the search: “Ford mustang trunk lock retainer”
https://www.ebay.ca/sch/i.html?_nkw=...k+retainer&_sa
That search will bring up a variety of trunk lock retainers for a Mustang. Hopefully it helps.
You can also try searching on LMR or NPD’s site too as they are the largest vendors of reproduction (and some NOS) Mustang parts.