Hey there PFCate,
Thanks for reaching out to Apple Support Communities. We're happy to provide some assistance.
This article can assist you on how to use the search feature in the Messages app:
Search for a conversation in Messages on Mac
In the Messages app
on your Mac, click in the search field in the top-left corner of the sidebar.
Do one of the following:
Choose a category: Select a conversation, photo, location, and more to quickly find a conversation.
Enter search criteria: Enter what you’re searching for (such as a word or phrase), then select a suggestion or press Return.
When you select a result, the conversation’s transcript opens at the location of the search result.
If you need further assistance, let us know. We are here for you.
Have a great day!