Below piece of code will give you physical path of an IIS website.
Import-Module WebAdministration
[string] $iisWebsiteName = 'Default Web Site'
$iisWebsite = Get-WebFilePath "IIS:\Sites\$iisWebsiteName"
if($iisWebsite -ne $null)
{
Write-Output "Website's physical path:"
Write-Output $iisWebsite.FullName
}
else
{
Write-Output "Website not found"
}