I'm not aware of a taller sissybar for the LS650...but it's fairly simple to make the factory one taller.  If you pull the stock pad off of it, you can mount your own in it's place.  If you have a piece plywood, some foam (if nothing else, buy a $20 memory foam pillow at wallyworld), and some vinyl, you can make your own to whatever specs you want, then just put it back on with screws through the pre-existing holes in the sissy bar.