Happs foam density (the amount of foam per cubic foot of foam) does not determine firmness what determines firmess is the compression of the foam which can range from about 15 to 40. This is called the IFD. You will have to special order a sofa from a manufacturer that has some cushion options. Look for an IFD of about 28. If the store won't tell you find out the name of the manufacturer and call them yourself and ask the manufacturer if they will special order a firmer cushion. Most companies of US made sofas will do this. They will probably charge extra to do this. I would guess about $200.

Your correct in that its important to have a high density foam or a HR foam for seating, but Freethinker99 is correct that the IFD is what determines how firm or soft the seat will be. Actually the costs of the foam is pretty much directly related to the density and the cost of the foam is the same for about all IFD's from 18 to 60.
But most manufacturers will charge a upgrade just to change to a firmer foam, even though the foam will cost them the same.
However their are some who offer free upgrade to a firmer seating.

Here is a link that might be useful: No up-charge for firmer seating