United States
A cluster of parking lots in downtown NYC are transformed on a Sunday to a giant outdoor antiques market, full of trash and treasure and people-watching opportunities.
I got an amazing silver necklace for $25 a few years ago, and there's always a few oddments that are good for people back home who hate tacky souvenirs but expect a gift.
Look for the tourist leaflets - the markets are clustered around 20-something street.
However cheap these may appear at the prevailing exchange rate, you should be aware that UK Customs will be expecting you to declare the item on arrival: if you do so, the additional tax and duty will most likely cancel out your "bargain". On the other hand, if you try to smuggle one in, not only will you will have to pay the tax and duty but a stiff fine as well (and you won't be getting out of Arrivals any time soon, either !).
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