Buy a curved upholstery needle from any sewing supply store along with heavy duty thread - Joanne's Fabrics, WalMart, whatever is in your area. Knot upholstery thread, take one stitch into back of sofa, draw needle through loop on back of button (should be a metal or fabric loop), take a few additional stitches into back of sofa. It's an easy fix, you just need the right tools.
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Try fixfault.com to fix chairs.