You can find all the parts and accessories to your product here: http://www.samsungparts.com/Products/Parts_and_Accessories/HWD450/Default.aspx
Not sure if you'll find it there, since it's a big part, but that's my best bet.
I bought a sound bar (only) off ebay. It works great, but didn't come with a wireless subwoofer. Is there a way to find a replacement or substitute for it?