It won't be easy. If you have the old one use it as a pattern. Buy exterior waterproof fabric. A home sewing machine may not be heavy duty enough, but give it a try. If not an upholstery shop can make it. Check on line first ghosthor replacement covers, may be cheaper in the long run.
Check amazon.com for cheap prices and a large selection. I like the fact that you can read reviews from people that have bought the product. I have changed brands many times due to these reviews and haven't been disappointed yet.
Hi Mj, If you're looking for shade only, you may want to consider a product called Sun screen fabric. I put this on my gazebo over 5 years ago and have had no issues at all with it. It has survived the harsh Florida sun and 2 hurricanes! It's inexpensive too! Here's a link where I purchased mine.
Thank you so much for the information, appreciate the help. Couldn't find a replacement, guess they don't make it anymore