The best open source alternative to Yahoo! Finance is Wealthfolio. If that doesn't suit you, our users have ranked more than 50 alternatives to Yahoo! Finance and three of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Yahoo! Finance are StockTicker and ticker.
Wealthfolio is a simple, desktop-based investment tracker. Your financial data is stored safely on your own computer Ditch the spreadsheets, forget about those pesky subscription fees, and no more worries about SaaS services playing around with your data.
StockTicker - A resizable stock ticker widget for your android home screen, written in Kotlin
