I do computer repairs at the discrete component level, but practically no "computer shops" do this kind of work so I wouldn't expect them to have diodes lying around, unless someone is cannibalizing them out of PSU and that's definitely an option as a donor but they are probably throwing old PSU away, not harvesting parts out.
Diodes have several specs but not a direct wattage rating. They are basically rated for max voltage, max continuous current (which reflects how much they heat up, and whether there is a heatsink for types that can mount to one) and other parameters that shouldn't affect your application.
The thing is, you can way overshoot these minimums for pennies and have a more reliable diode than if replaced with the OEM part that failed.
A 1N400 should be a 1 amp diode. The missing # on the end is usually the voltage rating but any in that 1N400 series would be well above the circuit voltage, unless there really is some electric motor on the same circuit producing large spikes. See what else is on the same fused circuit but this is probably overthinking things when they used a cheap jellybean part that failed.
