This is a great question to ask, it is a question I have often found myself asking due to the stunted economy. Recently I have had to make a few repairs to my home, one of which was a replacement of a few drafty and old windows. While researching some products for replacement windows I found how expensive it can be to purchase them. This is much the same issue you probably have ran into while trying to find windows to enclose your porch; they are expensive. If you have a bit of time to dig deeper and research you may find it helpful to search for a habitat for humanity in your area that sells such things. Also, you can check with your local hardware stores to see if there are any custom order windows or returns that can no longer be sold on the self as new; often times you will find these types of situations offer deep discounts for the buyer. This is how I ended up finding some windows to replace my damaged ones at a really good price. Hope that you have the same good luck.
I looked on the internet for dealers of free used windows in northwest Ohio and I also came up with a blank. Unfortunately whenever I searched for windows I kept finding websites regarding the computer program "windows". But I did find this interesting on a Sydney window cleaner website. It has an article on how to clean your windows like a proffesional, this might be useful if you find these elusive windows and they are in a bad condition. Keep us updated if you can find a source.