Per the U.S. Postal Service:
Those who [could not] attend the event but who wish to obtain the postmark may submit a mail order request. Pictorial postmarks are available only for the dates indicated, and requests must be postmarked no later than 30 days following the requested pictorial postmark date.All requests must include a stamped envelope or postcard bearing at least the minimum First-Class Mail postage. Items submitted for postmark may not include postage issued after the date of the requested postmark. Such items will be returned unserviced.
Customers wishing to obtain a postmark must affix stamps to any envelope or postcard of their choice, address the envelope or postcard to themselves or others, insert a card of postcard thickness in envelopes for sturdiness, and tuck in the flap. Place the envelope or postcard in a larger envelope and address it to:
United States Postal Service
NAPEX Station
Postmaster
PO Box 9998
McLean, VA 22102-9998
Customers can also send stamped envelopes and postcards without addresses for postmark, as long as they supply a larger envelope with adequate postage and their return address. After applying the pictorial postmark, the Postal Service returns the items (with or without addresses) under addressed protective cover.