Ready to wear and off the peg mean the same. You walk into a shop and buy the garment "off the peg:", and it is "ready to wear". Nowadays you often hear the French phrase "pret a porter" meaning ready to wear. If the garment is made specially for you, with measurings, fittings, etc, it is "made to measure" or "custom made". The old word for it was "bespoke" meaning spoken for, ordered in advance, but this word is rarely seen today. Ganbatte!