Unfortunately, we don't have a great answer to this. We have considered making cushions available for customers, but it's not the same without the sofa underside support and sinuous springs.
We use high-resilience foam in our sofa cushions, which means they are a bit on the firmer side to start, but they will soften a bit to about medium density after a few weeks. This provides a comfortable but supportive feel that prevents sagging and will help keep your cushions looking new for years to come.
The best we can do is try to describe the level of firmness. Ask if the customer has a particular model interested in. (check the product on the website and check the construction details below, but most of the models are a med density high resilience foam)
The high resilience foam will retain its level of comfort for many years, compared to some foams which can degrade quickly over time and become very soft.