There is no such thing. If you saw one, it was modified in a chop-shop (no pun intended!). Maybe you saw a Chrysler PT Cruiser convertible? Same body style, different manufacturer.
Anyway, if it indeed was an HHR, you'll have to get yours done at a chop shop. You can't buy one like that from the factory.
People that are saying SSR: No, they're not even close to the same. An SSR looks like a little truck convertible, nothing like the HHR.