I have seen rounded back chairs go with just a seat cushion and no back cushion or use a square (though thin) small companion fabric pillow.
Depending on the exact curviture of the back, I have seen curved back chairs use a square cushion. With use, the cushion can conform or will conform while in use. Here are a few examples.

{{!gwi}}



The link below may also be useful.
Browse online sites and go to stores with floor displays and see what they are doing with similar styles to yours. Then copy it using their cushion or find similar somewhere else.
Laura

Here is a link that might be useful: Cushions Galore

Thanks for the great incite laura. I actually never thought about how a pillow or square cushion might look against a rounded back. Your pictures were great. I also tried the link you gave me but like the other online cushion sites, everything is very high priced. I don't use my deck furniture enough to warrant more than paying about $30 per cushion. Thanks again.

I agree that you can make a lot of cushions work with what you have but just have to say that the photos shown are not using cushions with a square back. Yes, the sides and front are straight but the backs have a curve to them.

Have you looked at HD and Lowes? They often carry cushions for chairs and they usually have a rounded back. Also check places like Walmart, Target, etc.
I've linked another online site below. They have several cushions for around $30.

Here is a link that might be useful: Cushions