firm cushions?

[Globetrotter]

I need some firm cushions for my sofa (back issues) and I'm looking for something that can be covered with a pretty cover, so NOT this type of lumbar pillow:

http://tinyurl.com/43nvpj6

I'm having a hard time finding firm cushions - almost any shape is fine, as long as it can be covered with a standard cushion cover! Any ideas?