Sending money from UK to Federal Prison

I am looking for a way to send money to a Federal Prison from the UK.
Calling Western Union and asking for a quick collect service no longer is an option as WU no longer take payment over the phone. Online it doesn't seem possible, unless I'm missing a trick - which is likely.
Money gram doesn't appear to offer the service.
Money order? But how does this work internationally?
A solution to this would be massively appreciated!