IPSW files are meant to download when you need to update your iPhone, iPod touch or iPad. You download this IPSW file in iOS updates folder, whenever there is an update to your iOS device. You should know the location of these files because they are helpful to you when you are going to solve any problem or jailbreak purposes. You can access these files directly from their original locations, and here you will find where they are actually located.
If you are looking for Mac OS X, the location of IPSW file depends on the iOS device, but usually the file will be located in home directory. When you open the folder, you will see all the files, for example an iPhone would store its iOS update in ~/Library/iTunes/iPhone Software Updates location.
In windows, location of IPSW files depends on the Windows version, username, and the iOS hardware you are using. If your device is iPhone and user is X, the location for windows XP is ‘\Documents and Settings\username\Application Data\Apple Computer\iTunes\iPhone Software Updates’ and for Windows Vista & Windows 7, the location is ‘\Users\username\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates’. For Windows 7 user, C:/Users/username/AppData/Roaming/Apple Computer/iTunes/iPhone Software Updates is the location to find IPSW files.