You can purchase USD-COIN from these places
Go to Binance
To provide users with more secure and convenient block-chain asset exchange services, polymerization of global high-quality block chain assets, is committed to creating a world-class block chain asset trading platform.
Go to Kucoin.com
The People's Exchange
Go to Bitfinex
The largest and most advanced cryptocurrencies exchange
Go to Bitstamp
Bitstamp is run from the United Kingdom and is one of the most popular fiat to Bitcoin exchanges.
Go to Gemini
The first U.S. exchange licensed for bitcoin and ether trading.
Go to Kraken
Buy, sell and margin trade Bitcoin (BTC) and Etherum (ETH) in exchange with EUR, USD, CAD, GBP, and JPY.