Simple! You may purchase slipcovers for almost any cushion shape/design via the link here.
We'd like to note that the total cost of the custom cushion slipcovers would be found at the bottom of the page (above the 'Add to Cart' button). The price you see above the fabric selection panel is the 'base price' while the final cost, would be calculated based on the fabric you've selected and the general dimensions of your cushion.
Please see the below for a better illustration of the custom cushion slipcover order process:
If you're purchasing individual pieces to slipcover partial parts of your furniture, it would be best to get fabric samples to decide on the best color matches!