Check a scrap yard or a local upholstery shop.
You should be able to find a real good seat at a scrap yard, then just bolt it in.
An upholstery shop may be able to rebuild what you already have.
We have had good luck with both options.
It's common on these trucks.Get some stiff high density foam,remove the seat cover,and build it up underneath.You may want to do the lumbar area too,as the support sucks there as well.An upolstry shop could do it for you as well.
I drive a 1998 Dodge..same problem. I went out and picked up an orbus-form seat cover. it worked for me and only cost about $25 bucks CDN. Plus it has heat and Masage. It is not a "true" seat cover but it has a full bottom and a 3/4 back to it.