Which bank do you have an account at? These ATMs take out foreign currency cash from your foreign currency accounts or exchange within your HKD accounts and then dispense.
Hang Seng for example has a lot of these foreign currency ATMs at their main branch in Central, also inside the Causeway Bay MTR station. Plus many others. Citi has them at their branches in Central and CWB.
Google is your friend, put "foreign currency ATM" in the search box.