Step 1.
download organization short name from
Esri's entire customer list revealed ( ArcHub )
CSV file direct download from here
https://transparentgov.net/data/json2tree/hub_v1_created_before_2025_3_26.zip
or
https://transparentgov.net/data/json2tree/opendata_v3_created_2017_01_01_to_2025_03_29.json.zip
Step 2.
shortlisted Esri's customer who had an established GIS asset
https://transparentgov.net/data/json2tree/opendata_refined_2017_to 2025_Mar26.zip
https://transparentgov.net/data/json2tree/hub_refined_2017_to 2025_Mar26.zip
Step 3.
looking for organization short name column, copy the short name, pasted in google search
for example : I randomly pick a org-shor-name is
titusville
Step 4.
google search organization short name and 'arcgis rest", you will find the self hosted enterprise portal arcgis server url
search titusville arcgis rest
you will find the top result is what you looking for, self hosted enterprise portal arcgis server rest service link
https://gis.titusville.com/arcgis/rest/services
Step 5.
try another short name sample ADEC
Step 6.
google search adec arcgis rest
top 2 result is their self hosted arcgis server