A shonky shop is a second hand shop that sells very cheap goods that are often third or even fourth hand. Most of the clothing has in fact been supplied by a pawnshop where poor people have sold their clothes.
One such shop is run by Soon Shine Sun in Clay Lane. Despite the lack of quality, various denizens of Cockbill Street both regularly patronize such shops and take pride in the clothes. It is known that they are too proud to buy from the pawnshop. In fact, Sam Vimes in his younger days went off to join the Watch in a suit proudly purchased from the shonky shop.
Frank McCourt's autobiographical novel about leaving Limerick, Ireland, for the USA, Teacher Man, relates the story of young Frank getting his first suit for a job interview in the USA from the local shonky shop. This was in Limerick Parish in the poverty and emigration years of the 1950's. Limerick Parish is the old area of the city, a place sharing many important functional characteristics with the Shades. Even today the Gardai don't go there in singles. Frank's terrifying granny did the negotiating in the local shonky shop, and her description of the place and its owner could be right out of Pratchett!