Did you order you computer online? Buy it from an Apple Store? Somewhere else? How did you pay for it?
I know when I purchase things in an Apple Store, they email the receipt to me. They may also be able to access it for you if you go in (when they're not busy and ask very nicely) and reprint it. If you ordered it from say, Amazon, you should be able to go in and reprint some sort of purchase confirmation.
Bottom line, call or email where ever you got it from and ask them if they can get you another copy.
Or, we can all come over to your house and help you look! 😉
with the dispatch email i got for my 3G the other day it mentionned that i might not recieve the 'invoice' for a few days and that it would come in an email, which it did, yesterday.