ERPNext comes with 2 predefined configurations for Currency Exchange Rate providers: Frankfurter.dev and Exchangerate.host

Frankfurter.dev

This is a free exchange rate provider that tracks daily exchange rates from 84 central banks, covering 201 currencies back to 1948.

frankfurter.dev/v2/

v2 supports more currencies than v1. For the complete list, visit: https://frankfurter.dev/currencies/

frankfurter.dev/v1/

v1 supports a limited set of currencies. Available Currencies:

{
"AUD":"Australian Dollar",
"BGN":"Bulgarian Lev",
"BRL":"Brazilian Real",
"CAD":"Canadian Dollar",
"CHF":"Swiss Franc",
"CNY":"Chinese Renminbi Yuan",
"CZK":"Czech Koruna",
"DKK":"Danish Krone",
"EUR":"Euro",
"GBP":"British Pound",
"HKD":"Hong Kong Dollar",
"HUF":"Hungarian Forint",
"IDR":"Indonesian Rupiah",
"ILS":"Israeli New Sheqel",
"INR":"Indian Rupee",
"ISK":"Icelandic Króna",
"JPY":"Japanese Yen",
"KRW":"South Korean Won",
"MXN":"Mexican Peso",
"MYR":"Malaysian Ringgit",
"NOK":"Norwegian Krone",
"NZD":"New Zealand Dollar",
"PHP":"Philippine Peso",
"PLN":"Polish Złoty",
"RON":"Romanian Leu",
"SEK":"Swedish Krona",
"SGD":"Singapore Dollar",
"THB":"Thai Baht",
"TRY":"Turkish Lira",
"USD":"United States Dollar",
"ZAR":"South African Rand"
}

Exchangerate.host

This requires an Access Key. It has both free and paid service. Check their site for more details.