Although it isn't the "official" Hummer variety, check this site's very own accessory section:
<A HREF="http://www.h2source.com/store/" TARGET=_blank>
http://www.h2source.com/store/</A>
Here's a similar fuel door:
http://www.h2source.com/rw202-1.htm
The one you have pictured is a GM Accessory... you might check with
GMPartsGuy on this forum...