Did you misplace your original or did you never receive one? Did you get an email confirming your completion and certification? Maybe it is still in your inbox.
I keep a print copy in a file. I also keep the email with confirmation of my completion of all certs that I may need proof of in a separate folder in my email.
Keeping CECs in a folder is a good idea as well. Come tax time I have easy access to all the receipts for CECs right in my email as well. This is also helpful when it is time to renew my certs. And I keep a print copy of this as well just in case.