ANY old PC AT tower case should fit. Not so common, usually made out of steel, so they have mostly rusted. The spacers used in these are none metric, usually made of brass, and don't fit the later ATX cases (usually metric, bright zinc plated, slightly taller).
Genuine A3000 PSUs are very rare for a simple reason - Commodore bought the entire production run of power modulators. There are no spares (CBM retained none to begin with). So it is unlikely you will find a genuine A3000T PSU that is going spare. Even a broken one is valuable, as it will have the power connector that fits into an A3000 motherboard. Same deal as an A500 power supply, the power connector is still valuable.
Modding a PC PSU to do the same job is not that straightforward, as A3000 PSUs had a little extra timer output. You CAN mod a standard PC ATX PSU to power an A3000, but you have to miss out the purple (timer) inputs. Instead, there's a couple of timer jumpers on the motherboard to set either PAL, NTSC, or PSU based timer controls. You might be able to bring these connectors out to case mounted switches, not sure, never tried. Would be a very useful mod to have to hand.
There are also issues with fitting an ATX PSU into an AT style case. One work around is to mod an AT PSU case so that the ATX PSU sits inside. That does work with a little trial and error, and means the finished work is compliant with current safety standards.
