There is an eBay store that sells some antique QSL cards:
http://stores.ebay.com/QSL-Etc
Click on the "QSL postcards" link on the left.
I think he's going out of business in June 15.
73 - Jim AD1C
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Jim Reisert AD1C, 7 Charlemont Court, North Chelmsford, MA 01863
USA +978-251-9933, <jjreisert at alum.mit.edu>, http://www.ad1c.us