Go to this address:
http://phobos.apple.com/versions
[The site will be presented to you in XML. You might need to enable your browser to display XML. Also, you might need to View Source. Regardless, it is there.]
Scroll through the page until you find the appropriate link. Highlight the full address, then copy.
you want to look for a file similar to this: iPhone1,1_1.1.3_4A93_Restore.ipsw
In your browser's address, paste the link. The download should commence.
Once completed, move the download to wherever you want it, making sure that you REMEMBER where you saved it.