See "Springfield, Colo. hosts special Simpsons stamp event," KDVR May 5, 2009
The article concludes:
For those who cannot go to the Springfield, CO May 7th event, but who wish to obtain the special Springfield, CO pictorial Maggie Simpson postmark by mail, they should affix a Simpsons or other First Class stamp to each envelope/postcard of their choice, address the envelopes/postcards to themselves or others, place them in a larger envelope, addressed to:
SIMPSONS POSTMARK, POSTMASTER, 188 E 9th AVE, SPRINGFIELD, CO 81073-9998
After applying the postmark, the Springfield, CO Post Office will return the envelopes/postcards through the mail. There is no charge for the postmark. All orders must be sent by June 6, 2009.