Any AT PSU would be suitable for replacing the current PSU. It might be difficult to find one though, so ATX with ATX to Amiga adapter might be easier to find.
The floppy power connector is bog standard, so it will be on any PSU you find.
Yes it will probably be easier to find an ATX PSU nowadays, and you can use either a ATX to Amiga or ATX to AT adapter, or you can hack the wires & connectors for youself.
I'm not sure what type of power switch your tower has, but if it's the old AT type, you can probably connect the wires from it to the green wire (PSU ON) and one of the black wires (GND) of the ATX PSU connector.
For more info about hacking ATX PSU's for use with Amigas, you can take a look at this page:
http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_Power_supplies/body_amiga_power_supplies.html
