Visit Intercourse - it has at least two fabric stores, Zooks and the Old Country Store. The Old Country Store also houses a museum of old quilts, and they usually have a special exhibit there.
It's been a while since I've been, so I'm not sure what shops are still open. Burkholders was a good one that we visited, as was Sauders. I'm sure Lancaster County has a tourism bureau with lots of information.
Enjoy!
Janet