Any hardware store sells dimmers, iether as a clip-into-a-standard-2-hole-faceplate module, or as a complete faceplate with switch and dimmer fitted. They will screw into a standard existing switchplate or socketplate, to suit the size you already have. If you dont want to fiddle with the wall, you can get in-line dimmers attached to a cord. Instructions come with it. These are bod standard light dimmers, just ask them for a dimmer, any hardware store will help you.
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