If you want to find the ID of the first node with a specific Document Type, use this code by supplying the document type alias here (i.e. the method signature property "docTypeAlias").

Umbraco Razor - find page by document type alias not ID

public int GetFirstNodeWithDocumentAlias(string docTypeAlias)
{
  int locationsPageId = -1;
  DynamicNode searchPage = null;
  var nodes = new DynamicNode(-1).Descendants();
  foreach (DynamicNode node in nodes){
    if (node.NodeTypeAlias.ToLower().StartsWith(docTypeAlias.ToLower())) 
    { 
      searchPage = node; break; 
    }
  }      
  if(searchPage != null) {
    locationsPageId = searchPage.Id;
  }
  return locationsPageId;
}