We're sorry you're having this issue with your tent. While we don't have replacement rainflys for sale, we do have a couple of options that may be able to keep your tent in service. Depending on the damage to your window (i.e. is it leaking only at the seams? does it have a hole in it? is it cracked?) the steps you will want to take will differ. Here are some ideas to get you started:
You may be able to find a fabric repair service who could remove the window and replace it with a piece of tent fabric. Check out REI's Repair Services Page for information about Rainy Pass, who we recommend for any fabric or outerwear repair or service.
Additionally, we encourage you to check out this other thread, Replacement Rainfly: Half Dome 2 Plus, which has one users step by step instructions for how they applied new seam sealant and waterproof treatment to their rainfly and brought their tent back to life. Lastly, there is more really useful information in these two Expert Advice articles, Tent Care Basics and How to Repair a Tent.
Hopefully this helps get your tent back into service, thanks!
At REI, we believe a life outdoors is a life well lived.