Where I can find a place id on google maps

The place_id is in the SearchResults from any of the SearchRequests:

from the documentation:

{
   "html_attributions" : [],
   "results" : [
      {
         "geometry" : {
            "location" : {
               "lat" : -33.870775,
               "lng" : 151.199025
            }
         },
         "icon" : "http://maps.gstatic.com/mapfiles/place_api/icons/travel_agent-71.png",
         "id" : "21a0b251c9b8392186142c798263e289fe45b4aa",
         "name" : "Rhythmboat Cruises",
         "opening_hours" : {
            "open_now" : true
         },
         "photos" : [
            {
               "height" : 270,
               "html_attributions" : [],
               "photo_reference" : "CnRnAAAAF-LjFR1ZV93eawe1cU_3QNMCNmaGkowY7CnOf-kcNmPhNnPEG9W979jOuJJ1sGr75rhD5hqKzjD8vbMbSsRnq_Ni3ZIGfY6hKWmsOf3qHKJInkm4h55lzvLAXJVc-Rr4kI9O1tmIblblUpg2oqoq8RIQRMQJhFsTr5s9haxQ07EQHxoUO0ICubVFGYfJiMUPor1GnIWb5i8",
               "width" : 519
            }
         ],
         "place_id" : "ChIJyWEHuEmuEmsRm9hTkapTCrk",  // <----------------------------------------------
         "scope" : "GOOGLE",
         "alt_ids" : [
            {
               "place_id" : "D9iJyWEHuEmuEmsRm9hTkapTCrk",
               "scope" : "APP"
            }
         ],

You should be able to find a place ID here: https://developers.google.com/places/place-id#find-id

The above solution works in most cases but sometimes fails when you have some individuals using the same address as a business. In case you have exhausted the options from the API and manual search provided in the Google documentation Here Or here

You can use the manual suggestion provided at this page which has always worked for me https://www.launch2success.com/guide/cant-find-google-places-id-trick-finding-google-places-id/

You may want a Place ID Text search by searching the place name.

Your sample request may look like

https://maps.googleapis.com/maps/api/place/textsearch/json?query=fish+market+sydney&key="YOUR_API_KEY"

You will find the "place_id" in the return JSON String.