For some jailbreaks such as redsn0w you need to find the iOS firmware file on your computer to complete the jailbreak. This firmware file, known as an .ipsw, is likely already downloaded to your computer. No matter what iPhone, iPod touch or iPad you have the firmware should be in this folder.
Go to C:\Users\(username here)\AppData\Roaming\Apple Computer\iTunes\iPod Software Updates
Go to (username here)/Library/iTunes/iPod Software Updates/
Once you locate the .ipsw you are looking for you may want to move it to a safe place as iTunes tends to delete older firmwares when new updates come out and you never know when you might need it again.
If you still can’t find the firmware try downloading it.