Price the waste into the product and keep on keeping on.
Unless you are more environmentally conscious. In that case, you aren't going to find a lot of 24" banner that wasn't slit down from another bigger size where waste is being thrown out anyway. The best size, that isn't slit with waste, you are going to find is 30" in my experience. You could have a 54" or 60" slit but there will be waste still if you want 24".
Or just get 30" banner and after hemming it will be 28" and no one will notice it isn't 24" unless it is for a very size specific application, which in my experience is not usually the case for banners. You could even do a sale for 28" banners for the price of 24"! Maybe I got carried away with that last part. Happy Friday!