A custom Window Grate will cost some $$$. However, you may find a 'standard' size that's a little smaller or larger than what you need. And from there 1) you may be able to enlarge / reduce the size of the 'opening' you want to cover, or 2) depending on the material the grate is made from, you could have it 'cut' to size (doing it yourself if you can find out what it's made of.) I suspect in Option #2, that's all the Retailer is doing when you're ordering a custom grate.
Other than that, go to My Favorite WEB Search Engine and start reading. Maybe these companies have local retailers. My best to ya and hope this helps.