The R-SIG-Finance email list has a thread "Free financial data - equities, equity options and ETFs - for quantmod package (or other packages)" where the following sources are discussed:
Yahoo Finance
https://finance.yahoo.com/
https://eodhistoricaldata.com/
Tiingo
https://www.tiingo.com/welcome
Quantgo
https://www.quantgo.com/
IEX
https://iextrading.com/trading/market-data/
Barchart
https://www.barchart.com
Quandl
https://www.quandl.com/
iqFeed
https://www.iqfeed.net/index.cfm
Interactive Brokers (with brokerage account)
https://www.interactivebrokers.com/en/home.php
ActiveTick
https://www.activetick.com/activetick/contents/
IEX
https://iextrading.com/trading/market-data/
Barchart
https://www.barchart.com
polygon.io
https://polygon.io/stocks
TradingPhysics
http://www.tradingphysics.com/Feeds/AvailableFeeds.aspx
Yahoo Finance
https://finance.yahoo.com/
https://eodhistoricaldata.com/
Tiingo
https://www.tiingo.com/welcome
Quantgo
https://www.quantgo.com/
IEX
https://iextrading.com/trading/market-data/
Barchart
https://www.barchart.com
Quandl
https://www.quandl.com/
iqFeed
https://www.iqfeed.net/index.cfm
Interactive Brokers (with brokerage account)
https://www.interactivebrokers.com/en/home.php
ActiveTick
https://www.activetick.com/activetick/contents/
IEX
https://iextrading.com/trading/market-data/
Barchart
https://www.barchart.com
polygon.io
https://polygon.io/stocks
TradingPhysics
http://www.tradingphysics.com/Feeds/AvailableFeeds.aspx