How to tighten a sofa form? My leather sofa seats, the nouns where you sit, the cushioning have...

My leather sofa seats, the nouns where you sit, the cushioning have got soft and I be wondering if anyone knows a opening to either replace the cushioning inside of them or to know how to tighten the cushion, so they are firm again? I'm in Portland, OR...any suggestions for companies surrounded by that area would be great. Thanks
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Answers:    Cushions buy foam 1/2' bigger next wrap in bat the stuff that won't have the cushions sliding around inside the form covers.
As for the spring area buy (2) 1/4 " 2X4 ply or peg or anything not easy enough or one as long as the couch. The sinkedge will stop and assist bring firmness back . I did this to mine and I hold a brother who weighs 500 lb or more. I screwed the board to the frame so no sliding.