Can you replace inside foam for an Ashley sofa?

Sure, most fabric stores - like JoAnn's has the foam. It's usually about 4" high, comes in a long roll - they will cut it to size for you - and is usually green. There are several thicknesses of foam to choose from, but the green is usually the most dense. Sometimes they have pieces cut. And, if you go to JoAnn's - if there is one in your area - be sure to use a 1/2 off or 60% off coupon! A tip is to put the foam into a plastic bag, like a trash bag, before inserting it into the cushions. Put the opening of the plastic bag, with the foam inside of it, in the cushion first so that you can slide the plastic bag off and out when the foam is in place. Good luck.

JoAnns or most fabric stores. Also check online. They may have better deals and usually have the correct precut size and thickness. They also offer a variety of firmness.

Buy stuffing from a craft store. A.C. Moore, Michael's or Hobby Lobby.

Foam isn't inexpensive, but if you have a coupon, hooray!!! :) Hope you find what you want, and it's not costly!

Yeah, that would be nice, but I think you're going to have to spend a little bit of money. If there is a business near you that does reupholstering, or even an auto upholstery shop, you can ask if they have foam remnants in the size you need, and they may have some that they can sell you inexpensively.