Where do I find an exterior plumbing box?
Are you sure? You should have full access through the escutcheon plate for valve &amp; shut-offs. Access panels aren't actually there for re-plumbing, since the plumbing's good for 100-years.

Access panels were there in the old days because valves didn't have cartridges &amp; needed access for servicing &amp; for operating shut-off valves, not that much of anything had local shut-offs back then. You rarely see access panels anywhere anymore because of the took way-way too long technology.

Otherwise, if you want one get just a panel cover at the store so you can insulate the valve. Instead of possibly having no ability to insulate with a tight shallow box of some kind.