There is a problem with what you have in mind. First of all, because a postmark is made at the post office when something is mailed, it is illegal to re-use an already postmarked piece of mail (envelope or package) without obscuring the previous postmark. Second, if you did so, the old postmark would be marked out by the post office, and a new postmark with the current and new date of mailing would be applied. So, the letter would still show just when it was actually mailed.
I know this isn't what you hoped to hear, but, knowing this, you won't be wasting your time by trying to find an envelope with a postmark from a previous date.
I have seen letters on occasion that are in a postmarked envelope from a particular past date. Then that envelope has been placed inside a new envelope in an attempt to "prove" the letter was already mailed, but returned and is now being resent. Doing that is kind of "iffy," as most people realize it is an attempt to prove a previous mailing.
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