The term you are looking for the “stuffing” is BATTING. It come is many thickness. You can get this at a fabric store, or in a bag at Walmart’s. You may also need to buy FOAM, and you’d need to go to a foam or upholstery store to buy this. (It’s not cheap! unless you buy scrap pieces) Since you are thinking of doing the recovering yourself, I assume you are like me: You have no money to spare. I’m one of those people who just “goes for it” so I rarely use a pattern. I will tell you how I have gone about Slip covering an old piece of furniture. My suggestion: Get an old king size sheet at the thrift store (or whatever size you need to cover your piece of furniture. Take this sheet and use it to make a “PATTERN” so you know how much fabric to buy (That is IF you want to make your slip cover from scratch, OR if you are actually reupholstering it) Just drape the old sheet over the couch, cut around it and take it to your fabric store & they will tell you the best way to sew the fabric together. But they’ll need to know if you have cushions you need to cover too. (So do a drawing first to take with you) OR, even better, to keep money down you can use a nice sheet that you love the pattern, in a dark Hunter Green or Navy one or whatever to actually make the slip cover out of! Fabric comes in a few standard widths: 45 or 60 inches wide times however long you need it. IF you decide to make your slip cover and want to buy the fabric (ready made slipcovers are quite expensive. Go to your local Walmart. Most Walmart’s have one rack of ONE DOLLAR a yard fabric. This is the only affordable way to cover a large piece of furniture. (As fabric is usually around $5 plus dollars a yard for less expensive fabric) Check back at Walmart every week until you find a fabric you like. They get in different fabrics all the time. Or check your local Thrift stores. Make sure you can wash the fabric and it’s thick enough so the old pattern doesn’t show through the new fabric. (Dark colors work best) As I said earlier if you need “stuffing” it’s either “Batting” or Foam you need. But if your furniture is that far gone, you might want to buy some new, Used furniture on Craig’s list, at a used store, Garage Sale or out of an ad in the local newspaper, (Many people are moving and just want to get rid of their old perfectly, good, decient furniture so they don’t have to pay to dispose of it.) THEN re-cover that piece, it will probably cost much less and look better in the long run. Here’s my “Recipe” for a SUPER EASY slip cover. First take batting if you need to cover any “mushed” areas or a thin piece of foam might work better… Then take a nice large sheet that will cover your piece. Drape it over, then tuck it in the back and by the side arms. Make pretty folds and pleats. Pin the pleats to hold them temporarily. Take some matching Rope, thick Twine (or even ribbon) and wrap this around the base, under where you’d sit. Make a nice tie on the side or at the back, Then Cut the excess fabric around the bottom. (Leaving about an extra inch or 2 for a hem. Either take the sheet back off and hem on a sewing machine, or better yet, use a high quality fabric glue like “Unique Stitch” to hem the bottom. What makes this look good is the “Tucks” tuck the fabric behind cushions, under cushions, down the sides, and anywhere else you can,(you can use thumb tacks on the undersides.) also make nice “Pleats”. Think of this like “Wrapping a large Package with fabric” Do it neatly and you will be happy with it. This is also a great way to cover furniture for the summer months, or if children are coming to stay for a while & you don’t want to worry about your furniture getting soiled.
Hope this helps!