Currency Layer is a well known API service provider which collects updated and accurate exchange rates in a daily bases, from multiple sources, mainly Central Banks and other Forex/FX sources.

More information here https://currencylayer.com/faq

• How accurate is currencylayer forex data, and where does it come from?  (From CurrencyLayer)

  Our spot exchange rates are sourced from a very broad base of commercial sources and banks around the world, each of varying types and frequencies. The larger weighting particularly for major currencies is towards commercial sources as they more accurately reflect the trading occurring in the markets.

  In order to ensure the highest possible level of data coverage and accuracy, the currencylayer system relies on a very distinct “validation and fallback” algorithm, assigning different priorities to each data source and validating each forex rate as it enters our databases. If a source fails to deliver an accurate quote, the next highest provider is queried for this particular currency pair. This enables us to sort out possible inaccuracies and provide the vast majority of all spot exchange rates with a precision of six decimal places.