Could you find a 9 1/2" star block and double it? 9 1/2" blocks are fairly common but I don't know where you would find a 18 1/2" star. I wanted 24 1/2" blocks for a fast quilt and increased my 12 1/2" blocks quite successfully. If you use 9 1/2" star blocks based on a nine patch grid, (for example friendship star) and instead of each component being 3 1/2", your new larger block would be working with 6 1/2" squares. Many of the star blocks do work on a 9 patch grid format. Hope this works for you.