[Arran]

I don't know why a 5Y4 would be that expensive, I've bought good testing #80s for less then that, basically the same tube as a 5Y3/5Y4 but with a different base. In my case I am on the lookout for 5Y4GTs as they don't cook the eye tube cables used in Canadian RCA A23s and sets with similar chassis, the ST style ones do. You don't really need a NOS tube anyhow, used but testing good is fine with a rectifier, in fact they can even be a little weak and still work. In the past I have installed jumpers between pins to make a 5Y3 work in a 5Y4 socket, one of the plate pins does not line up and one of the filament pins does not line up.
If you go with a solid state substitute it might be a good idea to add a current limiting thermistor in series with the output, maybe also a fixed resistor too is the B+ is a little high. The directly heated tubes are almost instant on but still need to warm up, the thermistor simulates that.
Now if you want much more of a replacement problem try a Rogers/Canadian Majestic/Deforest Crosley radio with a pair of 2X3s or 2Y3s. Unless you are very lucky you will only find good used 2X3s, I've never seen a NOS one.
Regards
Arran