There should be plenty of sources for resistors such as that. However you might want to install one, if there's space of course, of a higher power rating.
The resistor is probably placed in series with the transformer. When you apply power, the resistor limits the inrush current. Moments later a relay should switch the resistor out of the circuit and connect the traffo directly to the mains.
The instantaneous power the resistor is forced to allow through it will be many times the long term capability of the component. But because it only has to survive this for 0.5 a second or so, it's usually not a problem.
Now either the resistor gave up the ghost or something else has gone wrong. Maybe the relay that switches the resistor out of the circuit has failed in some way. This would be potentially lethal to the resistor as now the entire current demands of the amp would be forced through it.
4.7R does seem a bit low of a value however, I would expect something a bit higher, what size is the transformer?. So perhaps the resistor isn't doing what I think it's doing. Nevertheless it might be worth figuring out exactly what the circuit is doing (It can't be that complicated) before moving forwards. If possible you want to make sure that this isn't going to happen again.
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