iPod ipsw firmware download that provided by Apple, when you do the relevant operation at your own risk.
A1421 / A1509
11A501 · iPod5,1
11A465 · iPod5,1
10B329 · iPod5,1
10B146 · iPod5,1
10B141 · iPod5,1
10A523 · iPod5,1
10A406 · iPod5,1