You can't I'm afraid.
The only PSU that will work with the VB is the offical Nintendo one (I doubt there were any 3rd party ones)
Let me explain, the Virtual Boy's power comes from a battery pack on the back of the controller. It's basically a little box which slips in and out and can be replaced by the AC adapter pak.
I haven't been able to find a AC adapter for my VB up until now (I haven't really looked that hard), but I don't really need one as my VB works fine on rechargable batteries... unlike my GBA 🙁