Step 1: Find a software firm with programming capabilities
Step 2: Ask them to write a search function for you
Step 3: Ask them to do a load of work for free
Step 4: Try not to get cross when they laugh in your face
Step 5: Give up and go to Google GSA
I assume what you mean; you want a Search function rather than an actual full-blown Search Engine itself - as mentioned above.
There are plenty of PHP based search functions you can download from the internet that should index a single site’s pages and make it searchable similar to a Search engine.