i highly recommend trying the reissues first to see if you can hear any difference from the stock tubes, before plonking down a few hundred bucks per tube for NOS.....
Wel audio is probably the only place you can find old mullards in Singapore.
on a side note, JJ's EL84s work VERY well in that amp. these are rated for higher plate voltages than normal EL84s, they they will last long in the ACs, which are known to eat up power tubes. Martin Electronics carries them.