One single place for all the iPod Touch firmwares of all possible models starting from iOS 4.1. the links provided below are stock/original firmwares and comes direct from Apple servers. Please do not ask for torrents, Google it yourself. Please note that restoring older firmware versions truly depends on available SHSH blobs saved for your device. Read the SHSH blobs Doc if you don't know what those are. This Doc will be updated whenever Apple release new version.
NOTE : IPSW links of older iOS versions are removed from doc. We will remove links of firmware that are no longer being signed by Apple. We will add them back if a new downgrade method is found.
iOS 4.1 (released on 3rd September 2010)
iPod Touch 2G : http://appldnld.apple.com/iPhone4/061-7937.20100908.ghj4f/iPod2,1_4.1_8B117_Restore.ipsw
iPod Touch 3G : http://appldnld.apple.com/iPhone4/061-7941.20100908.sV9KE/iPod3,1_4.1_8B117_Restore.ipsw
iOS 4.2.1 (released on 18th November 2010)
iPod Touch 2G : http://appldnld.apple.com/iPhone4/061-9855.20101122.Lrft6/iPod2,1_4.2.1_8C148_Restore.ipsw
iOS 5.1.1 (released on 27th April 2012)
iPod Touch 3G : http://appldnld.apple.com/iOS5.1.1/041-4300.20120427.WvgGq/iPod3,1_5.1.1_9B206_Restore.ipsw
iOS 6.1.6 (released on 21st February 2014) (for iPod Touch 4th Generation only)
iPod Touch 4G : http://appldnld.apple.com/iOS6.1/031-3211.20140221.Placef/iPod4,1_6.1.6_10B500_Restore.ipsw
iOS 9.3.5 (released on 25th August 2016)
iPod Touch 5G : http://appldnld.apple.com/iOS9.3.5/...1B3834D2D062/iPod5,1_9.3.5_13G36_Restore.ipsw
iOS 11.2 (released on 2nd December 2017)
iPod Touch 6G : http://appldnld.apple.com/ios11.2/0...B9DC24E03F/iPodtouch_11.2_15C114_Restore.ipsw
iOS 11.2.1 (released on 13th December 2017) (CURRENT VERSION)
iPod Touch 6G : http://appldnld.apple.com/ios11.2.1...FB2783B2/iPodtouch_11.2.1_15C153_Restore.ipsw

NOTE : IPSW links of older iOS versions are removed from doc. We will remove links of firmware that are no longer being signed by Apple. We will add them back if a new downgrade method is found.
iOS 4.1 (released on 3rd September 2010)
iPod Touch 2G : http://appldnld.apple.com/iPhone4/061-7937.20100908.ghj4f/iPod2,1_4.1_8B117_Restore.ipsw
iPod Touch 3G : http://appldnld.apple.com/iPhone4/061-7941.20100908.sV9KE/iPod3,1_4.1_8B117_Restore.ipsw
iOS 4.2.1 (released on 18th November 2010)
iPod Touch 2G : http://appldnld.apple.com/iPhone4/061-9855.20101122.Lrft6/iPod2,1_4.2.1_8C148_Restore.ipsw
iOS 5.1.1 (released on 27th April 2012)
iPod Touch 3G : http://appldnld.apple.com/iOS5.1.1/041-4300.20120427.WvgGq/iPod3,1_5.1.1_9B206_Restore.ipsw
iOS 6.1.6 (released on 21st February 2014) (for iPod Touch 4th Generation only)
iPod Touch 4G : http://appldnld.apple.com/iOS6.1/031-3211.20140221.Placef/iPod4,1_6.1.6_10B500_Restore.ipsw
iOS 9.3.5 (released on 25th August 2016)
iPod Touch 5G : http://appldnld.apple.com/iOS9.3.5/...1B3834D2D062/iPod5,1_9.3.5_13G36_Restore.ipsw
iOS 11.2 (released on 2nd December 2017)
iPod Touch 6G : http://appldnld.apple.com/ios11.2/0...B9DC24E03F/iPodtouch_11.2_15C114_Restore.ipsw
iOS 11.2.1 (released on 13th December 2017) (CURRENT VERSION)
iPod Touch 6G : http://appldnld.apple.com/ios11.2.1...FB2783B2/iPodtouch_11.2.1_15C153_Restore.ipsw