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

Frankfurter.app

This is free exchange rate provider. But, it provides exchange rates for 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.