@KA Thank you for your patience as we tried to see what we could find out for you!
Unfortunately, a replacement rainfly is not an item that is available to purchase on its own. One option would be to purchase a lightweight tarp like the REI Co-op Quarter Dome Tarp, but that would require a bit of work to figure out how to pitch and stake it out to be most effective.
When you say that your rain fly is deteriorating can you elaborate a little bit on that for me? Does that mean that the fabric is wearing out and becoming threadbare and/or has holes in it? Maybe the seams are coming undone? Or does it mean that water is no longer beading up on it and/or the seam sealing isn't working any more?
The reason I ask is that if it is a waterproofing/seam sealing issue it might be easily remedied with some after market products like these to add water repellency and retreat the seams. There are also many products available to repair holes, tears, and seams that can give your rain fly new life.
You also might find value in a conversation I had with another member of our community about a tent issue. You can find that conversation here.
Hope this helps, thanks for reaching out!