I don't know about Greatful Dead fabric, but I think most of the head shops sell something that you might could cut down. I've seen huge batik-style wall hangings, flags, etc - and you know, if you did something with large (like 15.5") blocks in batiks and then appliqued a gigantic GD logo on it, I bet that would come out pretty awesome. So, my suggestion is skip the fabric stores & go to the head shop.