HF-2 uses the ACA power adaptor.
If the ACA adaptor is unavailable due to being discontinued, you can use a 9V battery or power the HF-2 from an effect unit, such as the BOSS TU-3, with a DC OUT jack compatible with the PSA adaptor. Please refer to the related Q&A "Can I Use the PSA Adaptor with BOSS Pedals Specified for the ACA Adaptor?".