Use Business Source Ultimate to find articles from scholarly and trade publications. Enter search terms like "shadow economy" or "underground economy."

Global Financial Data contains time series of black market exchange rates for about 50 countries. Click the "data series" tab and search for "black market."

IZA is an independent research firm that focuses on labor markets. Articles and working papers are available on the firm's website. Click "search" and perform a keyword search for "shadow economy."

Search EconLit for "shadow economy." Use options on the left side of the results screen to narrow your results.

Search ProQuest One Business for "black markets." You can use the tabs on the results page to narrow the results by publication type.