For one, each device has a different iOS firmware update, unless you were referring to five iPad's of the same configuration, as there are different firmware's for Wifi-only, 3G AT&T and 3G Verizon.
Anyway, you could download iOS directly from Apple, rather than using iTunes Download&Update function. A list with the available iOS 4.3.3 downloads can be found in the following sticky:
http://www.ipadforums.net/ipad-hacking/24939-official-apple-ios-firmware-download-links-ipad.html
However, if you already have installed the update on one device, than the firmware can be found under
C:\Users\Your Account Name\AppData\Roaming\Apple Computer\iTunes\iPad Software Updates
This is assuming, that you are using Windows. Also Windows tends to empty that folder after a restart.
On a Mac, iTunes is supposed to store the firmware updates here:
Home Folder/Library/Application Support/iTunes/iPod Firmware