My most profound apologies, Janet. I thought I replied to your question, but obviously, I was wrong.
First of all - good for you in taking care of the cats that were dumped on your property. Not everyone would go the extra mile that you've done.
Secondly - I respectfully disagree with Hannah Riley. While cats are independent creatures, they most DEFINITELY need shelter, especially in the winter. Depending on where you live, it can make a huge difference in a cat's life whether they have proper shelter or not. There's a statistic Alleycat Allies puts out - outside, feral cats only live on average for 3-4 years. Most of this is due to NOT having proper food, fresh water AND proper shelter.
There are several places to get an igloo - you might try Amazon or PetCo. Also, make sure you provide fresh water & food on a daily basis.