For small lots and oneoffs in the US, I would check Marlin P Jones Associates (MPJA.com) out of florida. One of their specialties seems to be liquidating NOS electronics stuff (Much of which I assume is no longer in production). They are particularly useful (to me) for power supplies.
If you need a guaranteed supply (of power supplies), then you may have to ask. They have some more expensive PSU's which may be in production. But the really cheap walwarts and whatnot appear to be closeouts. Whatever is for sale today could be gone tomorrow.
It's not unusual to find power supplies like what you're asking about for 4-5.00. But shipping is usually 9 or 10.00, minimum. So take a moment to browse and maybe buy a couple extra psu or other items.