You can usually find county and agency ArcGIS REST/Services one of two ways:

1. Type into Google Greene County NY “REST/Services” and search for the index then copy and paste into QGIS ArcGIS REST tab or use with Leaflet
2. Type into Google Saratoga County NY Interactive Map, then open up Web Tools in your browser, click the Network tab on bottom and search for Network resources containing either the term MapServer or feature. Some might be the standard ArcGIS Aerial or Topographic layers but if you look carefully you can find the URL for the ARC MapServer or FeatureServer.