Not an uncommon problem. We (I) are thinking about doing this, but have no personal experience. I asked about this back in antiquity and was told "there are outfits that do that". There are custom sheet makers who can take your measurements for all sorts of strange beds, including round ones. Google them under custom bed sheets. For example, you might try looking at bedderbedding.net. Again I have no experience with this, so you are on your own. If you do it, please let us know how well you were satisfied.