Try some of the fabric search engines. You'll get more options if you search using 'guitar' than if you search using 'guitar panel'.
http://www.quiltshops.com/index.html
http://webstore.quiltropolis.net/
http://www.fabshophop.com/fabsearch.asp
The only panel I found was this one and each separate block is only 4¼" square, but there are lots of non-panel fabrics with guitars on them that you might be able to use.
http://www.fabricshack.com/cgi-bin/S...id=1#rjr2232-1