Sherrill furniture has many divisions: Hickory White, Whittemore-Sherrill, CTH Sherrill. They have many different seating options from firm to soft. If you want to contact the factory directly, google whitepage directory for Hickory NC and it should bring up their phone number. Just ask for customer service.
I cant post directly on this forum.
Their is also many websites that can make sofa cushions,give them your sizes and they can ship to you.

My Thomasville Mercer sectional had the same issues. When I bought it originally, the frame was webbed and you couldn't upgrade to spring down cushions. I found the hard cushions too hard, didn't want all down, and the comfort crown just right (i.e., comfortable). But with that comfort comes quick wear. After two years, the mainly used cushions are shot and basically pancake. when you sit on them. I complained to Thomasville (they have a 3-year warranty), and Thomasville sent out a technician within a week who approved me for replacement cushions. He told me that this type of cushion always fails after a couple of years and that it's the most popular cushion. Good news, however, is that Thomasville switched the frame to springs while mine was being assembled, so I can upgrade to spring down now (for extra money). So I'm doing that.

I've found the number for the factory and will give them a call on Monday. Thanks!

Next question, if I can get replacement cushions (either from them or another company) what do I need to get to avoid this in the future? I'm allergic to down, so that's not an option for us. We use this sofa as our main resting and relaxing area, so it gets tons of use.

Finally heard back and they said new cushions will eventually do the same thing. The salesperson suggested maybe seeing if an upholster could make foam core inserts?? Hubby says sell it and try again!

Thoughts from the experts please....

If you remove the seat cushions and sit on the deck under the cushions does that sag. You have to determine if its the cushions and or the suspension system. As far as the cushions if the manufacturer won't help you and I think they will, even if you have to pay for them, you could contact one of the cushion manufacturers in NC give them your measurements and see if they will make you a set of cushions. If you need help with their names post a note here. Any upholstery shop can also order cushions and I would buy a high quality cushion.

Freethinker99 is correct that you should determine if the seat is not sagging before replacing the cushions. If your looking for a very supportive seat cushion that doesnt soften very much then best to stay away from any seat cushions that have a polyester wrap or down and feathers. These types of wraps will always soften and flatten out after use. A high resilient 2.0 pound or higher density foam core with a ILD of 28 to 30 will provide a very supportive seat. If your cushions have a crowned center then you will need to wrap this core in a soft but also high density foam, 2.0 pound or higher. A soft low density foam will soften over time but a high density foam will not.

Quite a bit of engineering goes into a seat cushion so that the filling will fill out the cushions correctly, hold the correct shape and not break down and also when all the cushions are filled will fit in the sofa correctly. If you just give someone the measurements of the cushions, their is no guarantee they will fit correctly or hold up as well as they should.

I suggest contacting some local upholstery shops to come out and measure your sofa, take your cushions to their shop and make the fillings as I recommended; not using any polyester fiber wrap and a HR 2.0 pound higher density foam. He can determine if your cushions are crowned, meaning actually was cut & sewn and sized for a softer core wrap. Depending on where you live will determine the costs, but in the end you should have what your looking for.

Here is a link that might be useful: Choosing sofa cushion seating

A good quality cushion such as suggested by rmanbike can last 20 years or more and that probably is your best approach at this time if the manufacturer will not replace the cushions.