If you already have the Powershell code to restore one database file, you are nearly done. Simply do a Powershell command to list all of the backup files in your folder (wherever they are). Then pipe those file names into your restore script and replace the hard-coded file name in the script with the piped-in value.
If you don't have the backup script or wanted to look at another, here is a fairly good one:
http://www.sswug.org/articles/viewarticle.aspx?id=44909[
^]