I have been using the following URL for the past 3 years without issue. However, it has stopped returning results.
URL: https://query.yahooapis.com/v1/public/yql?q=s
Yes, it looks like Yahoo has discontinued their Currency Converter API service.
I found a different site : https://currencylayer.com/
They let you request currency rates (1 USD = 168 other currencies). If you make less than 1000 requests per month, it is free (if you need more, they have different subscriptions : https://currencylayer.com/product).
You just need to sign up and receive your own Access Code. Once you have that, then you simply make the call http://apilayer.net/api/live?access_key= and it will return JSON of the other currency rates.
They also have code examples in PHP, JavaScript, and Java at https://currencylayer.com/documentation
Found it very easy to get started using their API right away.