Find all template fields where the source is not empty using Sitecore PowerShell Extensions.

Report-Template-Sources-Faster.ps1

$watch = [System.Diagnostics.Stopwatch]::StartNew()

$path = "/sitecore/templates/Project"
$templateId = "{AB86861A-6030-46C5-B394-E8F99E8B87DB}"
$templateFieldId = "{455A3E98-A627-4B40-8035-E683A0331AC7}"

$db = Get-Database -Name "master"
$rootItem = $db.GetItem($path)
$templates = $rootItem.Axes.GetDescendants() |
    Where-Object { $_.TemplateId -eq $templateId } | Initialize-Item

$reportItems = [System.Collections.ArrayList]@()

foreach($template in $templates) {
    $templateFields = $template.Axes.GetDescendants() |
        Where-Object { $_.TemplateId -eq $templateFieldId } |
        Where-Object { ![string]::IsNullOrEmpty($_.Fields["Source"].Value) }
    
    foreach($templateField in $templateFields) {
        $reportItem = @{
            "Template Name" = $template.Name
        }
        $reportItem["ID"] = $templateField.ID
        $reportItem["Field Name"] = $templateField.Name
        $reportItem["Field Source"] = $templateField.Fields["Source"].Value
        $reportItems.Add([PSCustomObject]$reportItem) > $null
    }
}
$watch.Stop()
$watch.ElapsedMilliseconds
$reportItems | Show-ListView

Report-Template-Sources.ps1

$path = "/sitecore/templates/Project"
$templateId = "{AB86861A-6030-46C5-B394-E8F99E8B87DB}"
$templateFieldId = "{455A3E98-A627-4B40-8035-E683A0331AC7}"

$templates = Get-ChildItem -Path $path -Recurse | 
    Where-Object { $_.TemplateId -eq $templateId }

$reportItems = [System.Collections.ArrayList]@()

foreach($template in $templates) {
    $templateFields = Get-ChildItem -Path "master:" -ID $template.ID -Recurse |
        Where-Object { $_.TemplateId -eq $templateFieldId } |
        Where-Object { ![string]::IsNullOrEmpty($_.Fields["Source"].Value) }
    
    foreach($templateField in $templateFields) {
        $reportItem = @{
            "Template Name" = $template.Name
        }
        $reportItem["ID"] = $templateField.ID
        $reportItem["Field Name"] = $templateField.Name
        $reportItem["Field Source"] = $templateField.Fields["Source"].Value
        $reportItems.Add([PSCustomObject]$reportItem) > $null
    }
}

$reportItems | Show-ListView