Home button spring can be purchased on Ebay for $9.99
Home Button Spring for iPod Touch 2nd/3rd Generation
That was the only place I found that sells it.
The diagram for the home button is like this: http://imageshack.us/photo/my-images/215... In that image when the digitizer is in place 1 and 2 are in contact with each other and 3 and 4 are in contact with each other. The button just completes the final connection.
If you don't want to pay $9.99 then you can just solder a thin wire between pad 3 and pad 4... that's really all the spring is doing (connecting 3 and 4). The home button when pressed will make the final contact.