How can I have firmer seats on my sofa?

I have a leather and tapestry sofa which is filled with eiderdown stuffing whilst I am quite happy with the back cushions I cant stand the cushions which we sit on as they go constantly flat I was wondering if there was an alternative to stuff these. These sofas were purchased from Alan Ward's about 4 years ago, they did say at the time I could re stuff the cushions but I think I would prefer something more solid, can you help with an alternative filling