MOSCOW—The ruble is down more than 30% against the dollar and the Russian economy is headed for stagnation at best, according to official forecasts. What are many Russians doing? Going shopping.

“For me, the currency crisis yielded a new Brompton £100 ($160) cheaper than in Britain,” said Peter Favorov, a magazine editor in Moscow, referring to a British-made folding bicycle that costs about £1,000 in the U.K.

The...