Someone just asked this yesterday.
I sent them to
http://www.quiltshops.com/ - just put Batman in the search field - there were a ton of results.
Here are a bunch all in one shop:
http://equilter.com/cgi-bin/webc.cgi/st_search.html
And, I'm sure Joann's has a cheaper version. Seems like Batman would be a stock item with their novelty fabrics.