Jet.com offers the Jordan Manufacturing Sunbrella Rectangular Bench Cushion in Crimson Floral for $119.99 with free shipping. (Hayneedle has it for the same.) That's $30 off list and the lowest price we could find. It's covered in weatherproof acrylic and measures 48" x 18".
Note: You can also receive additional discounts during checkout by waiving free returns or by choosing select payment options.