If you go to Watford Valves web site they will steer you right. For new valves you could look at Mullard's, Groove Tubes or JJ's. The ECC83's are also 12ZX7's (the US catagorisation), ECC81's are 12AT7's, and EL84's are 6BQ5, though you don't see that name as much.
I would think that ECC83's are your preamp stages, the ECC81 the phase inverter. The EL84's are the power tubes and the GZ34 the rectifier valve.
The ones that make the most difference to the sound are the ECC83's and the EL84's.